Wax is typically parafin disolved in styrene. It foats to the surface of the patch and keeps the air off. Gel coat is air inhibited and will not fully cure on the surface while exposed. It will feel cured but when you go to sand it will quickly gum up your paper. If you have styrene, use it to thin the gel coat past, or striaght resin, the use of acetone greatly affects the cure. The prefered way to fill where you need thickened material is to use the original color gel coat and thicken with fumed silica to the desired consistancy. This will give the correct color and take the minimum of work to bring to the original finish. Another product that will be available for gelcoats and putties is called Patch Aid. It provides the same function as the wax but helps the cure even more. Its the way to go for secondary gel coat applications and patching