Pure White Gelcoat with Hardener - No Wax

Clean and sand the surface, then catalyze the gelcoat with MEKP and lay it down in two to three coats. Spray it at about 18 to 20 mils wet, or brush and roll thinner coats, letting each one flash before the next. Because polyester gelcoat is air-inhibited, the final coat needs surfacing wax (or a PVA film) to cure tack-free so you can sand and buff it.

Only with unwaxed gelcoat. Air stops the surface of polyester from curing, so unwaxed gelcoat stays tacky until you stir surfacing wax into the final coat, about 1/2 oz per quart (2 oz per gallon), or spray PVA over it. Leave the build coats unwaxed so they recoat without sanding, and wax only the last coat.

About 2% MEKP by volume at 77°F, which is about 19 cc per quart or 76 cc (about 2.5 fl oz) per gallon. Use a little more in a cold shop and a little less in heat, but stay between 1.5% and 2.5%. Measure with a graduated dispenser, since drops are not a reliable measure.

Waxed gelcoat has an additive that rises to the surface as it cures, sealing out air so the coat hardens tack-free and is ready to sand. Unwaxed (sanding) gelcoat has no additive, so it stays tacky until you add surfacing wax to the final coat or cover it with PVA. Use unwaxed when you plan to lay more coats, and use waxed for the last coat.

It is not recommended. Polyester gelcoat bonds well to polyester and vinyl ester laminates but adheres poorly to epoxy. If you must gelcoat over epoxy, let the epoxy fully cure, wash off any amine blush, sand it thoroughly, and expect a weaker bond. For epoxy repairs, a two-part polyurethane topcoat is usually the better finish.

Yes. For small repairs you can brush or roll gelcoat, though spraying gives the smoothest finish. Thin it about 5% to 10% with styrene so it flows out and self-levels, catalyze with MEKP, and add surfacing wax to the last coat. Work in thin coats to avoid runs, and plan to sand and buff afterward since some brush marks are normal.

A tacky surface almost always means air-inhibited cure, not too little catalyst. Unwaxed gelcoat will not harden on top until you add surfacing wax to the final coat or seal it with PVA. Other causes are too little MEKP, temperatures below about 60°F, or high humidity. Confirm your catalyst ratio, warm the shop, and wax or PVA the last coat.

At about 20 mil wet, one gallon of gelcoat covers about 80 square feet before losses. Allowing for overspray, plan about 60 square feet per gallon (about 15 per quart) when spraying, or about 70 square feet per gallon when brushing or rolling. Tooling and heavy color coats go thicker. The gelcoat calculator works it out for your job.

Thin polyester gelcoat with styrene, not acetone, at about 5% to 10% by volume. Styrene lowers viscosity so the gelcoat atomizes and flows out smoothly while keeping the resin chemistry intact, whereas acetone just flashes off and can leave the film weak and porous. Add the styrene before the MEKP, and stay at or below 10% so you do not weaken the cured coat.