Gel Coat Troubleshooting Guide: Diagnosing and Correcting Common Gel Coat Defects

Fibre Glast Learning Center | Technical Guides

Summary

A high-quality gel coat finish depends on much more than selecting the right material. Proper preparation, accurate catalyst ratios, consistent application techniques, appropriate environmental conditions, and controlled curing all contribute to a durable, attractive surface. When one or more of these variables changes, gel coat defects can occur.

Fortunately, most gel coat defects are both preventable and correctable. Successful troubleshooting begins by identifying the visible symptom, understanding its most likely causes, and following a step-by-step process to determine the appropriate corrective action.

This guide explains how to diagnose and correct many of the most common gel coat defects encountered during composite manufacturing, repair, and refinishing. Whether you are building a new composite part, restoring an existing gel-coated surface, or troubleshooting a cosmetic defect after curing, understanding the root cause of the problem is the first step toward a successful repair.

For guidance on selecting between gel coat and paint, see our Gel Coat vs. Paint: Choosing the Right Finish for Composite Parts. For detailed application procedures, equipment recommendations, and best practices, refer to our Gel Coat Application Guide: How to Apply Gel Coat for Professional Composite Finishes.

Key Technical Concept

Every Gel Coat Defect Has a Root Cause

One of the most common mistakes when troubleshooting gel coat is attempting to repair the visible defect without identifying what caused it.

For example, sanding away fish eyes may improve the appearance temporarily, but if silicone contamination remains on the surface, the defect is likely to return. Likewise, polishing a dull surface will not correct an improperly cured gel coat.

Successful troubleshooting requires identifying the root cause before selecting the corrective action.

In most cases, gel coat defects can be traced to one or more of four variables:

• Material – Gel coat formulation, catalyst ratio, additives, contamination, or compatibility.
• Equipment – Spray equipment, atomization, pressure settings, or worn components.
• Environment – Temperature, humidity, cleanliness, ventilation, or moisture.
• Technique – Surface preparation, film thickness, spray pattern, overlap, curing, or finishing procedures.

Rather than treating each defect as an isolated problem, experienced composite fabricators evaluate each of these variables until the root cause is identified.

In This Guide You'll Learn

This guide explains how to:

• Identify common gel coat defects by appearance.
• Understand the most likely causes of each defect.
• Determine whether a defect is cosmetic or potentially structural.
• Select appropriate corrective actions.
• Prevent recurring defects on future projects.
• Improve overall gel coat quality through proven best practices.

Inspect Before You Repair

Before attempting to sand, polish, recoat, or otherwise repair a gel coat defect, take a few minutes to evaluate the condition carefully.

Ask the following questions:

• Is the defect cosmetic or structural?
• Did the problem occur during application, curing, or after the part entered service?
• Is the defect isolated or present across the entire part?
• Does the defect affect only the gel coat, or does it extend into the composite laminate?
• Is the surface fully cured?

Answering these questions often eliminates many possible causes before any repair work begins.

Fibre Glast Technical Tip: Resist the temptation to immediately begin sanding or recoating. Spending a few minutes diagnosing the root cause often prevents repeating the same defect after the repair is completed.

Diagnostic Decision Framework

Experienced composite fabricators rarely diagnose defects by guessing. Instead, they narrow the possibilities using a logical process:

1. Identify the Visible Defect
2. Determine When It Occurred (Application, Cure, or Service)
3. Evaluate the Four Variables (Material, Equipment, Environment, Technique)
4. Confirm the Most Likely Cause
5. Correct the Root Cause
6. Repair the Surface
7. Implement Preventive Measures

Following a consistent diagnostic process reduces unnecessary repairs while improving the likelihood of permanently correcting the problem.

Quick Reference Troubleshooting Guide

The following table provides a starting point for diagnosing common gel coat defects. Detailed explanations and corrective actions for each defect are provided later in this guide.

Defect	Typical Severity	Most Likely Cause	First Thing to Check
Fish Eyes	Cosmetic	Surface contamination	Wax, silicone, oil
Pinholes	Cosmetic	Air entrapment or porosity	Mixing and application
Orange Peel	Cosmetic	Poor atomization or spray technique	Spray equipment settings
Runs & Sags	Cosmetic	Excessive film thickness	Wet film thickness
Dry Spray	Cosmetic	Gun too far from surface	Spray distance and pressure
Sticky Surface	Cosmetic	Air inhibition or improper cure	Wax additive or PVA
Under Cure	Moderate	Incorrect catalyst ratio or low temperature	Catalyst measurement
Cracking	Moderate	Excessive film thickness or cure stress	Film thickness
Print Through	Moderate	Resin shrinkage or laminate construction	Laminate schedule
Blisters	Moderate to Severe	Moisture intrusion or contamination	Surface condition
Poor Adhesion	Moderate	Inadequate surface preparation	Surface cleanliness
Delamination	Potentially Structural	Bond failure	Composite laminate

Understanding Severity

Not every gel coat defect requires the same response. Some affect appearance only, while others may indicate underlying issues that require additional evaluation.

Cosmetic
These defects primarily affect appearance and generally do not reduce the structural performance of the composite part. Examples include orange peel, fish eyes, dry spray, or minor runs.

Moderate
These defects typically require repair to restore durability, appearance, or environmental protection. Examples include cracking, print through, under cure, or localized blistering.

Potentially Structural
Some defects may indicate problems extending beyond the gel coat into the composite laminate. Delamination, widespread blistering, or impact-related damage should be evaluated carefully before cosmetic repairs are attempted.

Fibre Glast Technical Tip: Never assume every visible defect is only cosmetic. If reinforcement fibers are exposed or laminate damage is suspected, evaluate the composite structure before beginning cosmetic repairs.

How to Use This Guide

Each troubleshooting section follows the same format to help identify and correct defects efficiently.

For every defect, you'll find:

• What It Looks Like – How to recognize the defect.
• Typical Severity – Cosmetic, Moderate, or Potentially Structural.
• Most Common Causes – Ranked by likelihood.
• How to Confirm the Cause – Practical inspection techniques.
• Corrective Action – Recommended repair procedures.
• Prevention – Best practices to avoid recurrence.
• Fibre Glast Technical Tip – Practical advice based on decades of composite fabrication experience.

By following the same step-by-step approach for every defect, troubleshooting becomes more efficient, repairs become more effective, and future defects become easier to prevent.

Application Defects

Application defects occur before the gel coat has fully cured and are generally related to one or more of the four variables discussed earlier in this guide: material, equipment, environment, and technique.

Although these defects are usually cosmetic, they can affect the appearance, durability, and long-term performance of the finished composite part. Correcting the root cause before making repairs helps prevent the same problem from occurring again.

Fish Eyes

Typical Severity: Cosmetic

What It Looks Like

Fish eyes appear as small, circular craters or depressions in the gel coat where the material pulls away from localized areas of the surface. Rather than flowing uniformly, the gel coat separates around the contaminant, leaving small exposed or thin spots.

Fish eyes may appear individually or across large portions of the surface, depending on the source and extent of contamination.

Most Common Causes

Most Common

• Silicone contamination
• Wax residue
• Oil or grease
• Mold release contamination

Less Common

• Dirty spray equipment
• Moisture on the surface
• Airborne contaminants
• Incompatible surface cleaners

How to Confirm the Cause
Inspect both the affected and unaffected areas of the part. Consider the following:

• Was wax or mold release fully removed?
• Has silicone-based polish or lubricant been used nearby?
• Is contamination limited to one section of the mold?
• Has the spray equipment been cleaned thoroughly?
• Did the defect appear on multiple parts from the same mold?

Localized defects usually indicate surface contamination, while widespread defects often suggest contaminated equipment or improper cleaning procedures.

Corrective Action
Minor fish eyes can often be repaired by sanding the affected area, thoroughly cleaning the surface, and reapplying gel coat.

If contamination is widespread, completely remove the affected gel coat before correcting the source of contamination and recoating.

Do not attempt to simply spray additional gel coat over contaminated areas without first eliminating the cause.

Prevention

• Clean molds thoroughly.
• Use approved cleaning materials.
• Avoid silicone products near composite work areas.
• Inspect spray equipment regularly.
• Maintain clean mixing containers and tools.

Fibre Glast Technical Tip: Silicone contamination can be transferred from unexpected sources, including aerosol lubricants, automotive detailing products, hand lotions, and contaminated shop rags. Establish dedicated "silicone-free" work areas whenever possible.

Related Defects: Pinholes, Poor Adhesion, Blisters

Pinholes

Typical Severity: Cosmetic

What It Looks Like
Pinholes are very small holes or tiny voids visible in the cured gel coat surface. Although often difficult to see individually, large numbers of pinholes can reduce gloss, affect appearance, and allow moisture penetration over time.

Most Common Causes

Most Common

• Air entrapped during mixing
• Air trapped during spraying
• Porous substrate
• Excessive spray distance

Less Common

• Improper catalyst mixing
• Contaminated materials
• Applying gel coat too dry

How to Confirm the Cause
Inspect the defects under bright, angled lighting. Determine whether the pinholes:

• Occur uniformly across the surface.
• Follow reinforcement patterns.
• Appear only in repaired areas.
• Extend into the laminate.

Review mixing procedures to determine whether excessive air was introduced before spraying.

Corrective Action
Sand the affected area until the defects are removed. Clean thoroughly. Apply additional gel coat as required before sanding and polishing to the desired finish.

Large numbers of pinholes may require more extensive refinishing.

Prevention

• Mix slowly to minimize entrapped air.
• Maintain proper spray distance.
• Apply recommended film thickness.
• Avoid excessive atomization.
• Prepare porous repair areas properly before applying gel coat.

Fibre Glast Technical Tip: Entrapped air often begins in the mixing container, not the spray gun. Stir thoroughly but gently to achieve uniform catalyst distribution without whipping air into the gel coat.

Related Defects: Porosity, Dry Spray, Fish Eyes

Porosity

Typical Severity: Cosmetic to Moderate

What It Looks Like
Porosity appears as numerous small voids or interconnected pockets within the cured gel coat. Unlike isolated pinholes, porosity often extends below the immediate surface and may become visible after sanding or polishing.

Severe porosity can reduce moisture resistance and shorten the service life of the finish.

Most Common Causes

Most Common

• Excessive air entrapment
• Improper application technique
• Excessive film thickness
• Incorrect catalyst ratio

Less Common

• Material contamination
• High humidity
• Excessive exotherm

How to Confirm the Cause
Inspect cross-sections of the defect if practical. Determine whether the voids are:

• Surface only
• Throughout the gel coat thickness
• Associated with thick application areas

Evaluate spray technique and film thickness measurements.

Corrective Action
Minor porosity may be repaired by sanding and recoating. Severe porosity generally requires complete removal of the affected gel coat before reapplication. Correct the underlying application problem before refinishing.

Prevention

• Apply gel coat in multiple even passes.
• Maintain proper film thickness.
• Mix materials carefully.
• Follow catalyst recommendations.
• Maintain stable environmental conditions.

Fibre Glast Technical Tip: Applying gel coat too heavily often traps heat and air simultaneously. Building thickness gradually produces a denser, more uniform surface.

Related Defects: Pinholes, Runs, Cracking

Orange Peel

Typical Severity: Cosmetic

What It Looks Like
Orange peel produces a textured surface resembling the skin of an orange. Although gloss may remain high, the surface lacks the smooth appearance expected of a properly applied gel coat.

Most Common Causes

Most Common

• Poor atomization
• Incorrect spray pressure
• Spray gun too far from the surface
• Improper viscosity

Less Common

• Cold material
• Improper nozzle selection
• Inconsistent spray speed

How to Confirm the Cause
Review spray pressure, nozzle size, material temperature, spray distance, and spray pattern. Confirm all fall within the equipment and gel coat manufacturer's recommendations. Compare the finish with a properly adjusted test panel whenever possible.

Corrective Action
After the gel coat has fully cured, orange peel can often be corrected by progressive sanding, compounding, and polishing. Severe texture may require additional refinishing.

Prevention

• Maintain consistent spray distance.
• Use the proper spray equipment.
• Verify atomization before beginning production.
• Keep gel coat within the recommended temperature range.

Fibre Glast Technical Tip: Always test spray settings on cardboard or a sample panel before spraying the actual part. Small adjustments made before production can prevent extensive finishing work later.

Related Defects: Dry Spray, Runs, Uneven Gloss

Dry Spray

Typical Severity: Cosmetic

What It Looks Like
Dry spray produces a rough, dull, powdery surface caused by partially dried gel coat reaching the work surface before it has an opportunity to flow together. The finish often lacks gloss and may feel rough to the touch.

Most Common Causes

• Spray gun too far from the surface
• Excessive air pressure
• Material beginning to cure before application
• Environmental conditions accelerating solvent evaporation

How to Confirm the Cause
Review spray technique and inspect surrounding areas for overspray. Dry spray often appears first around edges or in areas requiring longer spray distances.

Corrective Action
Minor dry spray may be removed through sanding and polishing. More severe cases generally require sanding and recoating.

Prevention

• Maintain proper spray distance.
• Keep the spray gun perpendicular to the surface.
• Use recommended spray settings.
• Apply gel coat within its working time.

Fibre Glast Technical Tip: If the surface begins to look dry during application, stop and evaluate your equipment settings before continuing. Continuing with incorrect settings usually increases the amount of rework required.

Related Defects: Orange Peel, Uneven Gloss, Pinholes

Curing Defects

Curing defects occur when the gel coat does not polymerize as intended or when curing conditions create excessive stress within the coating. These problems are often influenced by catalyst ratio, film thickness, temperature, humidity, or application technique.

Because many curing defects share similar symptoms, careful diagnosis is essential before attempting repairs.

Sticky or Tacky Surface

Severity: Cosmetic
Most Likely Cause: Surface cure inhibited by oxygen exposure
Usually Repairable: Yes

What It Looks Like
The gel coat appears to have cured, but the exposed surface remains tacky or sticky to the touch. In most cases, the material beneath has cured properly, while only the outermost layer fails to fully harden.

This is most commonly seen when a laminating (non-surfacing) gel coat is used without adding a surfacing agent (wax additive) or without applying polyvinyl alcohol (PVA) to exclude air during curing.

Common Causes

Most Common

• Oxygen (air) inhibition
• A surfacing agent (wax additive) was not added when required.
• Polyvinyl alcohol (PVA) was not applied when needed.

Less Common

• Incorrect catalyst ratio
• Low shop temperature
• Improper mixing

How to Confirm the Cause
Determine whether the tackiness is limited to the surface or extends throughout the gel coat. If the underlying gel coat is hard while only the outer surface remains tacky, air inhibition is the most likely cause. If the entire coating remains soft, investigate catalyst ratio, temperature, and cure conditions.

Corrective Action
For air-inhibited surfaces, wash away any PVA (if used), allow the gel coat to complete curing as recommended, then sand and finish the surface. If the gel coat has not cured throughout its thickness, removal and replacement may be necessary after correcting the underlying cause.

Prevention

• Use surfacing agents (wax additive or Duratec High Gloss additive) when recommended.
• Apply PVA where appropriate.
• Measure catalyst accurately.
• Maintain proper shop temperature.

Fibre Glast Technical Tip: A tacky surface does not always indicate a failed cure. Determine whether the tackiness is limited to the surface before assuming the entire application must be removed.

Related Defects: Under Cure, Poor Adhesion, Wrinkling

Under Cure

Severity: Moderate
Most Likely Cause: Incorrect catalyst ratio or low temperature
Usually Repairable: Sometimes

What It Looks Like
The gel coat remains soft, rubbery, or easily indented long after the expected cure time. Unlike simple air inhibition, the material beneath the surface has also failed to develop its intended hardness.

Most Common Causes

Most Common

• Too little MEKP catalyst
• Low material temperature
• Low shop temperature

Less Common

• Poor catalyst mixing
• Expired catalyst
• Contaminated materials

How to Confirm the Cause
Inspect the entire thickness of the gel coat. Review catalyst measurement, batch records, shop temperature, and cure time.

Corrective Action
Minor under-cured areas may continue curing if environmental conditions improve. Severely under-cured gel coat generally requires removal before reapplication. Correct the underlying curing conditions before repeating the repair.

Prevention

• Measure catalyst accurately.
• Mix thoroughly.
• Maintain recommended temperatures.
• Verify catalyst shelf life.

Fibre Glast Technical Tip: Increasing catalyst beyond the manufacturer's recommendations is not an appropriate solution for cold shop temperatures. Instead, bring the material, the surface, and the work environment into the recommended temperature range.

Related Defects: Sticky Surface, Wrinkling, Poor Adhesion

Wrinkling

Severity: Moderate
Most Likely Cause: Applying additional material before proper cure
Usually Repairable: Yes

What It Looks Like
Wrinkling appears as ripples, folds, or distorted surface texture where the gel coat has shifted during curing.

Most Common Causes

Most Common

• Recoating before adequate cure
• Excessive film thickness
• Solvent entrapment

Less Common

• Excessive catalyst
• High exotherm

How to Confirm the Cause
Determine whether the wrinkled areas correspond to thick application zones or locations where additional coats were applied prematurely.

Corrective Action
Allow the gel coat to cure completely before evaluating the extent of the defect. Localized wrinkling may be sanded and recoated. Extensive wrinkling often requires complete removal of the affected area.

Prevention

• Follow recommended recoating intervals.
• Apply multiple thin passes.
• Maintain recommended film thickness.

Fibre Glast Technical Tip: More material is rarely the answer. Building gel coat gradually almost always produces a more stable cure than attempting to achieve full thickness in one heavy application.

Related Defects: Runs, Cracking, Under Cure

Alligatoring

Severity: Moderate
Most Likely Cause: Excessive cure stress or incompatible recoating
Usually Repairable: Usually

What It Looks Like
Alligatoring appears as an irregular network of cracks resembling reptile skin. The pattern often develops during curing or shortly after recoating.

Most Common Causes

Most Common

• Recoating over incompletely cured gel coat
• Excessive film thickness
• Cure stress

Less Common

• Material incompatibility
• Excessive catalyst

How to Confirm the Cause
Determine whether cracking is confined to the surface or extends into the laminate. Review the timing between coats and verify the gel coat system used.

Corrective Action
Sand away the damaged gel coat, correct the underlying cause, and reapply according to the manufacturer's recommendations.

Prevention

• Allow adequate cure between coats.
• Maintain recommended film thickness.
• Verify product compatibility.

Fibre Glast Technical Tip: Surface cracking caused by recoating too soon cannot usually be corrected by applying additional gel coat. Eliminate the unstable material before refinishing.

Related Defects: Cracking, Wrinkling, Poor Adhesion

Cracking

Severity: Moderate
Most Likely Cause: Excessive film thickness or cure stress
Usually Repairable: Yes

What It Looks Like
Cracks may appear as isolated hairline fractures or larger networks of visible surface cracks. Depending on the cause, they may remain cosmetic or indicate excessive stress within the gel coat.

Most Common Causes

Most Common

• Gel coat applied too thick
• Excessive exotherm
• Cure shrinkage

Less Common

• Impact damage
• Structural movement
• Improper laminate support

How to Confirm the Cause
Inspect whether the cracks remain confined to the gel coat or continue into the composite laminate. Measure film thickness whenever practical.

Corrective Action
Minor cosmetic cracks can often be repaired by grinding, sanding, and applying new gel coat. If cracking extends into the structural laminate, complete the structural repair before restoring the cosmetic surface.

Prevention

• Maintain recommended film thickness.
• Build thickness through multiple passes.
• Follow proper curing procedures.
• Avoid excessive catalyst levels.

Fibre Glast Technical Tip: Not every crack is a gel coat problem. If the crack follows structural reinforcement or impact damage, inspect the laminate before beginning cosmetic repairs.

Related Defects: Print Through, Alligatoring, Blisters

Appearance and Long-Term Performance Defects

Some gel coat defects do not become apparent until after the part has fully cured or entered service. These defects may result from application or curing problems, laminate construction, environmental exposure, or long-term service conditions.

Although many of these issues initially appear cosmetic, some can indicate underlying laminate problems that should be evaluated before cosmetic repairs are attempted.

Print Through

Severity: Moderate
Most Likely Cause: Resin shrinkage, laminate construction, or excessive cure stress
Usually Repairable: Sometimes

What It Looks Like
Print through (also called print-through or fiber print) occurs when the reinforcement pattern beneath the gel coat becomes visible on the finished surface. Depending on the laminate, the weave of fiberglass fabric, stitched reinforcement, or core transitions may appear as subtle surface texture or visible patterns.

Print through may develop immediately after curing or gradually become more noticeable over time as the laminate continues to stabilize.

Most Common Causes

Most Common

• Resin shrinkage during cure
• Reinforcement telegraphing through the gel coat
• Excessive exotherm
• Gel coat applied too thin

Less Common

• Laminate design
• Improper cure schedule
• Excessive post-cure temperatures

How to Confirm the Cause
Inspect whether the visible pattern corresponds to the reinforcement beneath the gel coat. Review gel coat thickness, laminate design and build, cure temperatures, and reinforcement type.

Corrective Action
Minor print through may be improved through sanding, refinishing, or applying additional surface coatings where appropriate. More severe print through often requires evaluation of the laminate design or manufacturing process before cosmetic repairs are attempted.

Prevention

• Maintain recommended gel coat thickness.
• Follow proper cure schedules.
• Minimize excessive exotherm.
• Design the laminate to minimize reinforcement print through by selecting appropriate reinforcement materials, ply sequencing, and surface veils where cosmetic appearance is important.

Fibre Glast Technical Tip: Print through is often a symptom rather than the root cause. Correcting only the surface appearance may not prevent the condition from returning if the underlying laminate continues to shrink or move.

Related Defects: Cracking, Uneven Gloss, Air Bubbles

Uneven Gloss

Severity: Cosmetic
Most Likely Cause: Inconsistent application or finishing
Usually Repairable: Yes

What It Looks Like
Certain areas of the surface appear dull while others maintain the expected gloss level. The difference may only become visible under reflected light or after polishing.

Most Common Causes

Most Common

• Uneven film thickness
• Inconsistent curing
• Improper sanding or polishing

Less Common

• Surface contamination
• Spray technique variations

How to Confirm the Cause
Inspect the surface using bright, angled lighting. Determine whether the dull areas correspond to repairs, overlap zones, or differences in film thickness.

Corrective Action
Most gloss variations can be corrected through additional sanding, compounding, and polishing. If gloss variation results from poor cure or contamination, additional refinishing may be required.

Prevention

• Apply consistent film thickness.
• Maintain uniform spray overlap.
• Follow recommended polishing procedures.

Fibre Glast Technical Tip: Inspect gloss under multiple lighting conditions. Surface variations often become much more visible in reflected sunlight than under overhead shop lighting.

Related Defects: Orange Peel, Dry Spray, Uneven Color

Uneven Color

Severity: Cosmetic
Most Likely Cause: Inconsistent film thickness or pigment distribution
Usually Repairable: Yes

What It Looks Like
The finished surface exhibits variations in color intensity or shading across different areas of the part.

Most Common Causes

Most Common

• Uneven gel coat thickness
• Incomplete mixing
• Variable spray overlap

Less Common

• Batch variation
• Different repair materials

How to Confirm the Cause
Review mixing procedures, material batches, spray pattern, and film thickness.

Corrective Action
Minor color variation may be corrected by refinishing affected areas. Significant differences often require recoating larger sections to achieve a uniform appearance.

Prevention

• Mix gel coat thoroughly before catalyzing.
• Maintain consistent application thickness.
• Blend repairs carefully.

Fibre Glast Technical Tip: Always intermix multiple containers of the same color when appearance is critical. This helps minimize normal batch-to-batch color variation.

Related Defects: Uneven Gloss, Dry Spray, Print Through

Chalking

Severity: Cosmetic
Most Likely Cause: Long-term ultraviolet exposure
Usually Repairable: Usually

What It Looks Like
The surface becomes dull, faded, or develops a powdery residue that can often be wiped away by hand.

Most Common Causes

• UV weathering
• Long-term environmental exposure
• Lack of regular maintenance

How to Confirm the Cause
Inspect exposed surfaces for oxidation while comparing protected areas that received less sunlight.

Corrective Action
Light oxidation can often be restored through compounding and polishing. More advanced weathering may require refinishing.

Prevention

• Wash surfaces regularly.
• Apply appropriate waxes where recommended.
• Store components out of prolonged UV exposure whenever practical.

Fibre Glast Technical Tip: Regular cleaning and polishing can significantly extend the appearance and service life of gel-coated surfaces exposed to outdoor environments.

Related Defects: Uneven Gloss, Blisters

Blisters

Severity: Moderate to Potentially Structural
Most Likely Cause: Moisture intrusion
Usually Repairable: Depends on cause

What It Looks Like
Blisters appear as raised bubbles or localized swelling beneath or within the gel coat surface.

Most Common Causes

Most Common

• Moisture intrusion
• Osmotic blistering
• Surface contamination

Less Common

• Trapped solvents
• Laminate defects

How to Confirm the Cause
Determine whether the blister is confined to the gel coat or extends into the laminate. Open a representative blister only if appropriate and inspect for moisture or laminate damage.

Corrective Action
Small cosmetic blisters may be repaired locally. Extensive blistering should be evaluated to determine whether moisture has affected the structural laminate.

Prevention

• Maintain complete cure.
• Prevent moisture intrusion.
• Follow proper laminate construction procedures.

Fibre Glast Technical Tip: Do not assume every blister is cosmetic. Widespread blistering may indicate deeper laminate issues requiring structural evaluation.

Related Defects: Delamination, Poor Adhesion, Cracking

Poor Adhesion

Severity: Moderate
Most Likely Cause: Inadequate surface preparation
Usually Repairable: Yes

What It Looks Like
The gel coat separates from the underlying substrate either during curing or after the part has entered service.

Most Common Causes

Most Common

• Improper surface preparation
• Surface contamination
• Incorrect recoating procedure

Less Common

• Material incompatibility
• Incomplete cure

How to Confirm the Cause
Inspect the failed interface carefully. Determine whether separation occurred between the gel coat and laminate, between gel coat layers, or between the repair material and original surface.

Corrective Action
Remove all loose material. Prepare the substrate properly before reapplying gel coat.

Prevention

• Clean thoroughly.
• Sand appropriately.
• Follow recommended recoating procedures.

Fibre Glast Technical Tip: Adhesion failures almost always begin before the gel coat is applied. Careful surface preparation remains one of the most effective ways to prevent costly repairs.

Related Defects: Fish Eyes, Delamination, Sticky Surface

Delamination

Severity: Potentially Structural
Most Likely Cause: Laminate bond failure
Usually Repairable: Requires structural evaluation

What It Looks Like
Delamination occurs when layers within the composite laminate separate from one another. Although it may first become visible as surface cracking or localized movement, the underlying problem extends beyond the gel coat itself.

Most Common Causes

Most Common

• Impact damage
• Poor laminate bonding
• Moisture intrusion

Less Common

• Manufacturing defects
• Long-term fatigue loading

How to Confirm the Cause
Inspect for movement, hollow sounds during tap testing, or visible separation between laminate layers. Determine whether reinforcement fibers have been damaged.

Corrective Action
Structural repairs should be completed before restoring the cosmetic gel coat surface. Do not rely on gel coat repairs to restore structural integrity.

Prevention

• Follow proper laminate construction procedures.
• Maintain adequate consolidation.
• Protect finished parts from impact damage.

Fibre Glast Technical Tip: Gel coat is a protective and cosmetic surface, not a structural reinforcement. Always repair the composite laminate before restoring the gel coat finish.

Related Defects: Blisters, Cracking, Poor Adhesion

Repair, Recoat, or Start Over?

Once the root cause of a gel coat defect has been identified, the next step is determining the most appropriate corrective action. While some cosmetic defects can be corrected through sanding and polishing, others require recoating or complete removal of the affected gel coat.

Attempting the wrong repair can increase labor, waste materials, and, in some cases, make the defect more difficult to correct. Evaluating the severity and extent of the damage before beginning repairs helps ensure the best long-term results.

When Sanding and Polishing Are Appropriate

Many cosmetic defects can be corrected without applying additional gel coat. Typical candidates include:

• Minor orange peel
• Light dry spray
• Slight uneven gloss
• Minor surface scratches
• Light oxidation or chalking

After the gel coat has fully cured, progressive sanding followed by compounding and polishing can often restore a smooth, high-gloss finish.

Fibre Glast Technical Tip: Begin with the least aggressive abrasive capable of removing the defect. Removing more gel coat than necessary can shorten the service life of the finish and increase the risk of exposing the underlying laminate.

When Recoating Is Recommended

Recoating is often appropriate when the existing gel coat remains well bonded but the cosmetic defect cannot be corrected through finishing alone. Examples include:

• Localized fish eyes
• Pinholes
• Minor porosity
• Uneven color
• Localized cracking confined to the gel coat
• Small repair areas

Before recoating:

• Remove damaged gel coat.
• Feather repair edges.
• Clean the surface thoroughly.
• Prepare the substrate according to the manufacturer's recommendations.
• Verify compatibility between the existing surface and the new gel coat.

Proper surface preparation remains one of the most important factors affecting successful adhesion.

When Complete Removal Is Necessary

Some defects indicate that localized repairs are unlikely to provide a satisfactory or durable solution. Complete removal should be considered when:

• The gel coat has failed to cure properly throughout its thickness.
• Extensive contamination has affected large areas.
• Widespread cracking is present.
• Adhesion failure affects multiple locations.
• Structural repairs are required beneath the gel coat.
• Multiple previous repairs have created inconsistent surface quality.

In these situations, correcting the underlying cause before reapplying gel coat generally produces better long-term results than attempting repeated cosmetic repairs.

Cosmetic vs. Structural Damage

One of the most important decisions during troubleshooting is determining whether the problem affects only the gel coat or extends into the composite laminate.

Cosmetic defects typically include:

• Fish eyes
• Orange peel
• Dry spray
• Uneven gloss
• Minor pinholes
• Light chalking

Potential structural concerns include:

• Delamination
• Impact damage
• Laminate cracking
• Moisture intrusion beneath the laminate
• Extensive blistering associated with laminate degradation

When structural damage is suspected, complete the structural repair before restoring the cosmetic gel coat surface.

Fibre Glast Technical Tip: Gel coat provides a durable, protective, and cosmetic finish, but it is not a structural reinforcement. Always evaluate the laminate before repairing the surface.

Document Your Findings

When troubleshooting recurring gel coat defects in a production environment, documenting the root cause and corrective action can help prevent similar issues on future projects.

Consider recording:

• Gel coat manufacturer and product
• Catalyst percentage
• Shop temperature and humidity
• Material temperature
• Wet film thickness
• Spray equipment settings
• Cure time
• Corrective action taken

Keeping consistent process records makes it easier to identify trends and improve manufacturing consistency over time.

Fibre Glast Technical Tip: Many recurring gel coat defects are process-related rather than material-related. Maintaining simple production records often reveals patterns that would otherwise go unnoticed.

Frequently Asked Questions

What is the most common cause of gel coat defects?

Most gel coat defects can be traced to one or more of four variables: material, equipment, environment, or technique. Surface contamination, incorrect catalyst ratios, improper film thickness, inconsistent spray technique, and inadequate surface preparation account for the majority of defects encountered during both manufacturing and repair. Because multiple variables often contribute to a single defect, identifying the root cause before beginning repairs is more effective than treating the visible symptom alone. Evaluating which of the four variables changed before the defect appeared is usually the fastest path to a correct diagnosis.

Why is my gel coat still sticky or tacky after it has cured?

If the tackiness is limited to the exposed surface, the most likely cause is oxygen inhibition. This is normal behavior for polyester gel coat cured in open air without a surfacing agent (wax additive) or a PVA (polyvinyl alcohol) release film. In this case, the gel coat beneath the surface has typically cured properly, and the tacky surface does not indicate a catalyst, mixing, or cure failure.

If the gel coat remains soft or tacky throughout its full thickness, the cause is different. This typically indicates an incomplete cure due to insufficient MEKP catalyst, poor catalyst mixing, low material or shop temperature, expired catalyst, or other conditions that prevented proper polymerization. In these cases, the affected gel coat may need to be removed and reapplied after the underlying cause has been corrected.

What causes fish eyes in gel coat?

Fish eyes are almost always caused by surface contamination. Silicone is the most common source, and it can be transferred from aerosol lubricants, automotive detailing products, hand lotions, polishing compounds, or contaminated shop rags even in small amounts. Wax residue, oil, grease, and mold release residue are also frequent causes. The gel coat pulls away from the contaminated spots during application, leaving small circular craters or thin areas. Localized fish eyes typically point to surface contamination in that zone; widespread fish eyes across the entire part usually indicate contaminated spray equipment or improper cleaning procedures. The contamination source must be eliminated before recoating, otherwise the defect will return.

What causes pinholes, and how are they different from porosity?

Pinholes are small individual holes visible on the cured gel coat surface, most commonly caused by air entrapped during mixing, excessive spray distance, or a porous substrate. Porosity refers to a broader condition where numerous interconnected voids exist within the gel coat, often extending below the visible surface, and is more commonly associated with excessive film thickness, heavy application technique, incorrect catalyst ratios, or high exotherm during cure. Both defects can reduce moisture resistance and surface appearance over time. Pinholes are usually addressed by sanding and recoating the affected area; porosity may require more extensive removal of the affected gel coat depending on the depth and distribution of the voids.

Why do I have orange peel on my gel coat surface?

Orange peel is caused by poor atomization, meaning the gel coat droplets are not broken down finely enough before reaching the surface, so they pile up into a textured finish rather than flowing together smoothly. The most common contributing factors are incorrect spray pressure, excessive spray distance from the surface, improper nozzle selection, or gel coat that is too cold or too thick for the equipment being used. Inconsistent spray speed can also produce orange peel in isolated bands across the part. Minor orange peel can usually be corrected after full cure through progressive wet sanding, compounding, and polishing. More severe texture may require refinishing. Testing spray settings on cardboard or scrap material before applying gel coat to the actual part is one of the most reliable ways to prevent the defect.

What causes dry spray, and can it be repaired?

Dry spray occurs when gel coat droplets partially cure before reaching the work surface, landing as a rough, powdery, low-gloss deposit rather than a smooth wet film. It is most commonly caused by holding the spray gun too far from the surface, using excessive air pressure, or continuing to spray after the material has begun to advance toward the end of its pot life. Dry spray often appears first around edges, recesses, or areas where gun-to-surface distance increases. Minor dry spray can usually be removed through sanding and polishing. More severe cases typically require sanding and recoating. If dry spray is observed during application, stopping to evaluate and correct the spray settings before continuing will reduce the total amount of rework required.

What causes gel coat wrinkling?

Wrinkling appears as ripples, folds, or surface distortion and is most commonly caused by applying additional gel coat before the previous coat has adequately cured, or by building excessive film thickness in a single heavy application. When fresh material is applied over an incompletely cured surface, the solvents in the new coat can soften the layer beneath and cause it to shift or buckle. Solvent entrapment within a thick application can produce a similar effect as the outer surface skins over while the interior remains fluid. Localized wrinkling can sometimes be sanded and recoated after the material has fully cured. Extensive wrinkling generally requires complete removal of the affected area before reapplication.

What causes alligatoring, and how does it differ from cracking?

Alligatoring is an irregular network of cracks resembling reptile skin that typically develops during curing or shortly after recoating. It is most commonly caused by applying new gel coat over an incompletely cured existing surface, excessive film thickness, or material incompatibility between successive coats. The pattern is broader and more irregular than the hairline fractures associated with standard cracking. Cracking, by contrast, tends to produce more defined lines and is most commonly associated with excessive film thickness, high exotherm, cure shrinkage, impact damage, or structural movement in the laminate beneath. Both defects require removing the affected material and correcting the underlying cause before refinishing. Attempting to apply additional gel coat over either condition without eliminating the root cause typically results in the defect returning.

What causes gel coat cracking, and when is it a structural concern?

Gel coat cracking most commonly results from excessive film thickness, high exotherm during cure, cure shrinkage, or impact damage to the part in service. Cracks confined to the gel coat layer are typically cosmetic and can be repaired by grinding, sanding, and applying new gel coat after the cause has been addressed. Cracking becomes a structural concern when the fractures extend beyond the gel coat into the composite laminate, when they follow the pattern of underlying reinforcement fabric, or when they are accompanied by laminate movement or delamination. Impact-related cracking that radiates from a focal point, sometimes called spider cracking, should always be evaluated for laminate damage before cosmetic repairs are made. Repairing only the surface without assessing the laminate will not prevent the cracks from returning if underlying structural movement continues.

What causes print through, and can it be corrected?

Print through occurs when the texture or weave pattern of the reinforcement fabric beneath the gel coat becomes visible on the finished surface. It is most commonly caused by resin shrinkage during cure, which pulls the gel coat surface down into the fabric texture as the laminate contracts. Gel coat applied below the recommended thickness range is more susceptible because the thin surface layer has less material to bridge the reinforcement pattern. High exotherm during lamination and certain laminate constructions can also contribute. Minor print through can sometimes be improved through sanding and refinishing or by applying additional surface material. More pronounced conditions may require evaluation of the laminate design or manufacturing process, because correcting only the surface will not prevent the condition from returning if the underlying laminate continues to shrink or move.

What causes gel coat blisters, and when are they a structural concern?

Blisters appear as raised bubbles or localized swelling within or beneath the gel coat and are most commonly caused by moisture intrusion. Blistering can also develop gradually when water seeps through the gel coat over time, collects at the surface of the laminate beneath, and builds enough internal pressure to push the gel coat upward. Surface contamination during application, laminate voids, and trapped solvents can also produce blistering. Small blisters confined to the gel coat layer can often be repaired locally by opening the blister, allowing it to dry thoroughly, and recoating after proper preparation. Widespread blistering, blisters that extend into the structural laminate, or blisters accompanied by delamination should be evaluated for deeper moisture damage before cosmetic repairs are attempted. Repairing the surface over a laminate that has absorbed significant moisture will not produce a durable result.

For applications involving prolonged water immersion, vinyl ester gel coats are often preferred because their molecular structure provides greater resistance to water permeation than conventional polyester gel coats. It is also why complete cure matters: under-cured gel coat leaves more unreacted compounds in the laminate that can dissolve and accelerate the process. And it is why prompt repair of chips and scratches is important, a break in the gel coat gives water a much faster path in than permeation through intact material.

What causes poor adhesion between new gel coat and the existing surface?

Poor adhesion almost always originates during surface preparation rather than during application. The most common causes are surface contamination from wax, oil, silicone, or mold release residue that prevents the new gel coat from bonding to the substrate; sanding that leaves the surface too smooth for reliable mechanical adhesion; and recoating over gel coat that has not fully cured. Material incompatibility between the new and existing materials can also contribute. Once adhesion failure occurs, all loose material must be completely removed before the surface is re-prepared and recoated. Simply applying additional gel coat over a poorly bonded area will not correct the underlying problem and typically results in a larger area of failure over time.

Why is my gel coat peeling?

Gel coat peeling is almost always an adhesion failure, meaning the new gel coat never bonded reliably to the surface beneath it. The most common cause is surface contamination: wax, mold release residue, silicone, oil, or grease left on the substrate prevents the gel coat from making proper contact with the surface, and the bond fails once the material cures and begins to experience normal stress. Inadequate sanding is the second most frequent cause; gel coat applied to a surface that is too smooth has nothing to grip mechanically, and even material that appears to cure well can peel away cleanly under relatively little force. Recoating outside the recommended window, either too soon before the previous coat has adequately cured, or too long after it has fully hardened without re-preparing the surface, also produces weak interfaces that fail over time. In some cases, the gel coat and substrate are simply incompatible, which is most commonly seen when polyester gel coat is applied over an epoxy laminate or an existing paint film without proper preparation or a compatible bonding system.

Peeling that affects a small localized area usually points to contamination or a preparation problem in that zone. Peeling that occurs across large sections of the part typically indicates a more systemic issue with the preparation process, the recoat timing, or material compatibility. In either case, all loose and peeling material must be completely removed before the cause is identified and corrected. Applying new gel coat over an area that has already failed without addressing the root cause will produce the same result.

What is delamination, and how does it differ from a gel coat surface defect?

Delamination is a separation between layers within the composite laminate itself, and is distinct from gel coat surface defects that remain confined to the finish layer. It is most commonly caused by impact damage, poor laminate bonding during manufacturing, moisture intrusion into the laminate, or long-term fatigue loading. Delamination may first become visible as surface cracking, localized movement of the gel coat, or hollow sounds when the part is tapped. Because delamination involves the structural laminate rather than the cosmetic surface, gel coat repairs alone cannot correct it. Structural repairs must be completed before the cosmetic surface is restored. Applying gel coat over a delaminated laminate without addressing the underlying separation will not restore structural integrity and may obscure damage that requires more extensive repair.

How do I know whether a gel coat defect is cosmetic or structural?

Cosmetic defects are confined to the gel coat layer and do not affect the structural performance of the composite part. Examples include fish eyes, orange peel, dry spray, uneven gloss, pinholes, and light oxidation. Structural concerns arise when a defect extends beyond the gel coat into the composite laminate. Signs that a defect may be structural include cracks that follow reinforcement patterns or penetrate through the gel coat into the laminate, exposed reinforcement fibers, hollow sounds when tapping the surface near damaged areas, delamination or visible layer separation, widespread blistering accompanied by laminate softness, and impact damage at a focal point. When structural damage is suspected, the laminate should be evaluated and repaired before any cosmetic gel coat work is performed. Completing cosmetic repairs over an unresolved structural issue will not prevent the defect from returning.

Should I sand, recoat, or completely remove damaged gel coat?

The appropriate corrective action depends on the severity, extent, and root cause of the defect. Sanding and polishing alone are appropriate for cosmetic defects confined to the surface layer that do not require additional material, such as minor orange peel, light dry spray, slight gloss variation, minor surface scratches, and light oxidation. Recoating is appropriate when the existing gel coat is well bonded and sound but the defect cannot be corrected through finishing alone, such as localized fish eyes, pinholes, minor porosity, small cracked areas, and localized color variation, provided the surface is properly prepared and the root cause has been corrected. Complete removal is necessary when the gel coat has failed to cure throughout its thickness, widespread adhesion failure is present, large areas have been contaminated, structural repairs are required beneath the surface, or multiple prior repairs have created an unstable or inconsistent substrate.

What is the best way to prevent gel coat defects?

Most gel coat defects are preventable when the four key variables (material, equipment, environment, and technique) are properly controlled throughout the application process. Before mixing catalyst, verify that the mold or repair surface is thoroughly clean and free of wax, silicone, oil, and moisture. Measure MEKP catalyst accurately using calibrated equipment and never estimate. Confirm that both the material and work environment are within the manufacturer's recommended temperature range. Apply gel coat in multiple thin, even passes to build the recommended film thickness of 18 to 24 mils rather than attempting full coverage in a single heavy coat. Use a wet film thickness gauge to verify coverage during application. Inspect spray equipment before starting production and test atomization on cardboard before spraying the actual part. Addressing preparation, measurement, and technique consistently from the start of each project significantly reduces rework and improves the quality and durability of the finished surface.

Conclusion

Successful gel coat troubleshooting is not simply about recognizing defects, it is about understanding why they occurred and taking the appropriate corrective action. By approaching each problem systematically and evaluating the four key variables (material, equipment, environment, and technique), fabricators can identify root causes, make effective repairs, and reduce the likelihood of recurring issues.

Many gel coat defects are cosmetic and can be corrected through proper sanding, polishing, or localized recoating. Others may indicate deeper problems involving the laminate, curing process, or manufacturing procedures that require more comprehensive evaluation. Distinguishing between cosmetic and structural issues is an essential part of successful troubleshooting.

When combined with proper application techniques and sound manufacturing practices, systematic troubleshooting helps produce more durable, attractive, and reliable composite parts while reducing costly rework and unnecessary downtime.

Successful troubleshooting begins long before defects appear. Consistent mold preparation, accurate material measurement, proper application techniques, and controlled curing conditions remain the most effective ways to produce high-quality gel coat finishes while minimizing rework. Investing time in prevention is almost always more efficient than correcting defects after they occur.

For additional guidance, explore the other resources in Fibre Glast's Gel Coat Learning Series and our broader Learning Center covering composite materials, mold construction, repair techniques, resin systems, and fabrication best practices.

Continue Your Composite Learning

Gel Coat Learning Series

• Gel Coat vs. Paint: Choosing the Right Finish for Composite Parts
• Gel Coat Application Guide
• Gel Coat Troubleshooting Guide

Composite Fundamentals

• Composite Materials Guide
• About Reinforcements
• Choosing Between Polyester, Vinyl Ester, and Epoxy Resins

Mold & Tooling Resources

• Plug Construction Guide
• Mold Construction Guide
• Plug Surface Preparation & Mold Surface Maintenance
• Step One Mold Polishing Guide and Step Two Mold Polishing Guide
• Moldless Composite Construction

Composite Repair & Manufacturing

• Ultimate Fiberglass Repair Guide
• Vacuum Bagging Composites
• Hot Bonder Guide

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About This Technical Guide

This guide is part of the Fibre Glast Learning Center, a growing collection of educational resources developed to help engineers, manufacturers, fabricators, technicians, and repair professionals better understand composite materials, fabrication processes, and industry best practices.

Since 1957, Fibre Glast has supplied high-performance composite materials and technical expertise to customers throughout North America and around the world. Our goal is to provide practical, technically accurate information that helps customers select the right materials and achieve successful results.

This guide is provided for educational purposes. Always consult the technical data sheet, product instructions, and safety information supplied by the manufacturer for the specific materials being used.

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