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 if you are using unwaxed (sanding) gelcoat. Unwaxed gelcoat stays tacky on the surface because air stops polyester from curing, so you add surfacing wax to the final coat, or spray PVA over it, to seal out air and cure it hard for sanding. Gelcoat sold with wax already contains it and cures tack-free on its own.

Catalyze polyester gelcoat at about 1% to 2% MEKP by volume, with 1.25% a good starting point near 70°F. That works out to roughly 1.5 to 2.5 ounces of MEKP per gallon of gelcoat. Use a little less in hot weather and more when it is cold, and always follow the ratio printed on your MEKP. Do not exceed about 3%, which can crack or discolor the coat.

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.

As a rule of thumb, one gallon of gelcoat covers about 45 to 50 square feet at the 18 to 20 mil thickness a durable finish needs, so a quart covers roughly 12 square feet. Tooling and heavy color coats go thicker, and spraying loses some to overspray, so order 10% to 20% extra for the job.