***This post was taken from fiberglasswarehouse.com with their permission***
There are two types of gel coats- General Purpose (GP) Gel coat and Tooling Gel coat.
General Purpose (GP) Gel coat is more flexible and softer than tooling gel coat. It is used where the end product will be submitted to constant vibration, such as boats and car bodies. It will flex and not crack under a constant beating. It can be scratched easier than tooling gel coat.
Tooling Gel coat is harder and less flexible than GP gel coat. It is mostly used on molds. It is harder to scratch which is ideal for molds that will be used numerous times.
Gel coat is a polyester resin and uses MEKP as the catalyst. Mix in MEKP at 1% to 2%. The increase or decrease of MEKP will effect the pot-life and cure time of the gel coat. Too little catalyst will take too long to cure. Too much catalyst and the gel coat may pull away from the mold due to shrinkage.
To thin the gel coat, use styrene monomer. It is possible to use acetone to thin gel coat, but there is a chance the gel coat will be rubbery or soft. Styrene, on the other hand will promote cross-linking of the resin and catalyst. Do not add more than 10% styrene or acetone by volume. Thinning the gel coat gives it a better flow when spraying. It also helps reduce brush marks when brushing.
Spraying is the best way to apply gel coat. Spray guns are available specifically made for gel coat. The guns are designed to work with the more viscous gel coat.
*If you will use your own paint spray gun, be prepared with acetone to clean the gun immediately after you finish spraying, or else the gel coat will harden in the gun. If this happens, you will wish you had purchased a gun specifically for gel coat. At least a 2 mm nozzle is needed. Also, your regular paint spray gun may not hold up to the chemicals found in gel coat.
Brushing gel coat is possible, but the finish quality is far inferior to spraying. Brushing can create a more brittle finished product. The styrene in gel coat does not evaporate as readily as it does when sprayed. Do not brush on gel coat the same way you would paint. Try using long, even strokes instead of short quick strokes. Use an unpainted, wood handled brush with natural bristles.
You can add pigments to the gel coat to match repairs or to make different colors. You add the pigment before the catalyst.
*** Working with fiberglass and resin is more of an art than a science. The information on this page is designed to be helpful. Experiment to see what works best for you before starting your project.***