Learning Center

*** 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.***

There are two types of gelcoats, General Purpose (GP) Gelcoat and Tooling Gelcoat.

General Purpose (GP) Gelcoat is more flexible, softer and easier scratched than tooling gelcoat. It is used were 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.

Tooling Gelcoat is harder, less flexible than GP gelcoat. It is used were you don’t want movement, such are molds, bathtubs and countertops. It is harder to scratch which is ideal for molds that will be used numerous times.

Mixing

Gelcoat 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 gelcoat. Too little catalyst will cause you to wait too long to put the second coat on and increase the chances of contamination. Too much catalyst and you will have to worry about the gelcoat pulling away from the mold due to shrinkage. Click here to see our catalyst quantity page to determine how much MEKP to add.

To thin the gelcoat, use styrene. It is possible to use acetone to thin gelcoat, but there is a chance the gelcoat 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.

Application

Spraying is the best way to apply gelcoat. Spray guns are available specifically made for gelcoat. The guns are designed to work with the more viscous gelcoat.

*If you want to use your own paint spray gun, be prepared with acetone to clean your gun immediately after you finish spraying, or else the gelcoat will harden in the gun and you will wish you had purchased a gelcoat gun. At least a 2 mm nozzle is needed. Also, your regular paint spray gun may not hold up to the chemicals found in gelcoat.*

Brushing gelcoat is possible, but the finish quality is far inferior to spraying. Brushing can create a more brittle finished product. The styrene in gelcoat does not evaporate as readily as it does when sprayed. The extra stryrene will cause it to be more brittle. Do not brush on gelcoat the same way you would paint. Try using long, even strokes instead of short quick strokes. Use a natural bristle, un-painted wood handled brush.