Proper gel coat application is difficult for professionals and is probably the most common problem area for those new to fiberglass work. Most common gel coat problems are related to temperature variation, catalyzation, and a variety of handling techniques. The following list of gel coat problems and causes should help to resolve some of the difficulties associated with gel coat.
A coating less than five mils thick may wrinkle, especially when brush marks are present. Check the thickness using a gel coat thickness gauge. The preferred thickness is 0.010 to 0.020 inches.
A wrinkle can also occur if the gel coat is not cured enough prior to lay-up. Before lay-up, check the surface for tack. The surface should be sticky but not transfer to your finger. Several things can affect a slow cure: solvent or water entrapment, under-catalyzation, and cold temperatures, among others.
If the gel coat film contains pinholes, check the spray equipment for moisture in the air lines or dirt in the traps. It is also important to review your spray techniques.
Any temperature below 70°F will retard the gel time. An under-catalyzed gel coat will also result in a slow cure but this is not a recommended technique for deliberately lengthening gel time. High moisture and humidity will lengthen the gel time.
Excessively heavy application, in one or more passes, is the most common reason for gel coat sags. To avoid sagging, spray in multiple light coats at a 15-inch distance from the mold. The maximum thickness to be sprayed at one application is 0.016 inches.
Gel coat porosity (air bubbles) is normally caused by trapped air or moisture. This can be prevented by limiting the spray thickness per pass to 0.005 inches. Be sure to verify proper air pressure—an over-pressurized spray can cause fine porosity. Air pressure should be set based upon the thickness of the gel coat (generally 40 to 80 psi, depending on viscosity). It is also important to avoid any spray overlap areas.
Most color separations are related to improper spray techniques. Reduce or lower thinning agents and lower the per-pass thickness of the gel coat. Again, avoid spray overlap.
This problem is caused from porosity related to air entrapment while spraying. This can be overcome by spraying the mold surface with several light passes. It is also important to bleed any moisture from the air lines and to be sure that the mold surface is dry.
These surface blemishes can be caused by improper resin-to-catalyst ratios and improper air pressure, both high and low. Another cause for craters would be oil or moisture on the surface of the mold.
Fisheyes are usually caused by contaminates on the mold like dirt, moisture, or oils. Bleeding the air lines and using air filters will lower any air-related contaminates. Try to isolate the spray area from any oils, especially silicones.
Blisters can be caused for the following reasons:
This condition is usually related to shrinkage of the gel coat. Shrinkage can be caused by:
This visual pattern is caused by inadequate gel coat thickness or improperly cured gel coat.
This condition is caused by an improper release of the mold surface. It is important to use release agents formulated for the reinforced plastics industry.
Proper training and common sense will eliminate most of the problems associated with gel coats. Proper handling of gel coats will result in a part with a blemish-free surface.