Gel Coat Troubleshooting Guide

Gel Coating

Gel Coat Spraying
Photo credit IStock Photo.

A proper gel coat application can often mean the difference between the look of a professionally fabricated part, and an amateurish attempt. Gel coating is one of, if not the most common problem area when working with composites, and can be a struggle for even experienced fabricators.

So how do you best avoid potential pitfalls when working with gel coats? First, be sure to follow proper procedure when spraying. Most gel coats will have information on proper catalyzation ratios to follow, be sure to consult your product data sheet before you begin mixing. Having improper catalyzation can cause slow gel coat curing, craters or pock marks, or blisters in the gel coat surface.

Also make sure that you are following the proper procedure for spraying when applying your gel coat. Most guns carried at Fibre Glast should be held perpendicular to spraying area, with a distance of about six to twelve inches, depending on atomization pressure and spraying conditions.

Once spraying, move the gun in several parallel lines until desired thickness is achieved. Problems can arise if your gel coat layer is too thick or too thin, make sure you a applying around .005 inches per pass, and avoid overlapping areas while spraying. The maximum amount of thickness for one application is .016 inches. If you are unsure on your coat thickness, use a Gel Coat Thickness Gauge as a guide. One gallon of gel coat will cover approximately 40-80 sq ft.

Improper temperature and humidity can also cause problems in your gel coat. Try to keep your work area at or above 70°F and free from moisture when working with gel coats. Cold temperatures can lead to slow Gel Coat Cure times, while moisture can cause pinholes, blisters and other imperfections in the gel coat.

Helpful Products

Item Why?
Spray Guns Spray guns allow for the most even application for gel coats during the application process.
Gel Coats Fibre Glast carries a wide selection of Gel Coats to choose from to fit the needs of your project.
MEKP MEKP is a popular choice for hardener when working with gel coats. Look for specific product information for your gel coat in order to find the correct mixing ratios.
Gel Coat Thickness Gauge Using a Gel Coat Thickness Gauge can help you avoid potential pitfalls associated with incorrect amounts of Gel Coat being applied during the spray process.

Gel Coat Complications

Below, you will find some of the most common gel coat problems that fabricators run in to, the reasons they might arise, as well as how to avoid them during your gel coat applications.


Wrinkles can occur in a gel coat if the application is less than five mils thick, especially when brush marks are present. To avoid this, make sure that you are reaching a preferred thickness of 0.010 to 0.020 inches. You can easily check this via a gel coat thickness gauge or similar tool.

These can also occur if the gel coat is not properly cured prior to lay-up. Before you begin, the surface of your gel coat should be sticky, but not transfer any material to your finger.

If you find that your curing process is progressing slowly it could mean that there is solvent or water entrapment, under-catalyzation, cold temperatures affecting the process, or other problems. Be sure to create a professional environment for the curing process, and follow proper procedures for gel coat application.


Pinholes appearing in your gel coat film can be caused by air pockets or dirt getting into your gel coat layer. Before spraying, check your equipment for moisture in the airlines as well as dirt in the traps. This problem could also be caused by improper spray techniques, so take the time to review your procedure for errors before you start.

Slow Gel Coat Cure

There are a number of reasons for a slow cure time, including temperature, under-catalyzation, and high levels of moisture and humidity. In order to avoid this, make sure that you keep the temperature of your gel coating area above 70°F, allow for proper catalyzation and keep the area free of moisture and humidity while the gel coat cures.

Sagging of the Gel Coat

Gel Coat Sagging
Photo credit IStock Photo.

If your gel coat appears to be sagging, it could mean that you have applied excessively heavy applications in one or more of your passes while applying the coat. The maximum amount of thickness to be sprayed in one application should be 0.016 inches and should be sprayed in multiple light coats at a 15-inch distance from the mold.


Air bubbles trapped in the gel coat layer is normally caused by trapped air or moisture. This may be due to incorrect levels of air pressure during the coating process. Too much pressure can cause fine porosity throughout your gel coat.

To prevent this, make sure to set proper pressure, based on the thickness of the gel coat and the viscosity (typically, 40 to 80 psi.) Also make sure to limit the spray thickness per pass to around .005 inches, and avoid overlapping areas while spraying.

Separation of Color

Separation of color is usually related to improper spray techniques during the coating process. To avoid this, consider reducing or lowering thinning agents and lower per-pass thickness of the gel coat. Once again, try to avoid spray overlap during your coating.

Discoloration on Finished Parts

A discoloration on your finished product could mean that air was trapped in the gel coat during the coating process. Be sure to keep your passes light, in order to avoid this.

Remember to strive for .005 inches per pass while spraying in order to avoid problems. You will also want to make sure to bleed any moisture from the airlines before spraying, and make sure that the mold surface is dry before you begin.

Craters and Pock Marks

Craters and Pockmarks are surface blemishes that could be signs of improper resin to catalyst ratios, or improper air pressure during the spray process. These blemishes can also be caused by oil or moisture on the surface of the mold.


Gel Coat Fish Eyes
Photo credit IStock Photo.

Fisheyes refer to small dots covering the surface of your gel coat, which are usually an indication of contaminates on the mold. This could be dirt, moisture or oils.

Before you begin the spraying process, make sure your mold has a clean, dry surface in which to work. Bleeding the airlines and using air filters can also help to lower any air related contaminates.


Blisters refer to soft, sagging areas on the gel coat, which could be an indication of a few different problems with the gel coat. Most commonly, this might point to an incomplete cure time. Remember, any temperature below 70°F will retard the gel time. Under-catalyzed gel coats, high moisture and humidity can also contribute to a lengthened cure time.

These blisters might also be an indication of improper wetting of the back-up fibers, a bad bond between the gel coat and back-up laminate (which can be an indication of contamination.)

Finally, blisters can be an indication of thin gel coat layers during the spray process.

Backup Pattern Shown Through Gel Coat

Seeing the backup pattern through your gel coat layer could either be an indication of inadequate thickness in the gel coat, or improperly cured gel coat.

Gel Coat Sticking to Part

If you find that your gel coat seems to be sticking to your part, this could be the result of an improper release of the mold surface. Make sure that you are using a release agent formulated for use within the plastics industry.

A proper gel coat can be the difference in any fabricators project, offering a blemish-free surface to your mold. By keeping a professionally controlled work area and following the proper procedures for your materials and tools, you can make sure to avoid many problems that can arise during the gel coating process.

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