Gelcoat does not fully cure without first supplementing it with a surfacing agent or wax additive sanding aid. Gelcoat may be thinned for use in a Portable Preval Sprayer with styrene monomer. You can spray it with a polyvinyl alcohol (PVA) before gelcoat reaches its gel stage (5-10 min.), or adding 1oz per quart of a wax additive sanding aid.
PVA is soluble in water making it easy to wash off. Applying can be achieved by spray or wiping the green liquid on with a sponge. It will foam slightly but dry to a clear, glossy shine. It can take 15-30 minutes depending on temperature.
For spraying gelcoat our FGCI brand does not recommend thinning.
If thinning is needed, the most cost effective and basic option is Styrene. Generally, you should not add more than 10% Styrene by volume. Patch Booster is an additive that will thin out gelcoat and not affect colors. This is added at 20-25% and the gelcoat will require catalyst at 2% so purchase an extra tube and omit wax (sanding aid).
Another thinning alternative is the Duratec High Gloss Additive. This is used with gelcoat for many reasons. Mixed at a 50/50 ratio this will produce a high gloss finish and will serve as a thinner for your gelcoat, that is why this is commonly used when spraying gelcoat. In addition, you’ll omit any additional sanding aid. After waiting a full 24 hours you can buff when cured, for a hi-gloss use 600 or higher grit paper and buff with Aqua Buff 2000 compound. Purchase an extra hardener as your gelcoat will need to be catalyzed at 2%.