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 for gun/nozzle used then Patch Booster is a additive that will thin out gelcoat and not affect colors, such as acetone or styrene. 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 extra hardener as your gelcoat will need to be catalyzed at 2%.