when working with gel cote if it does not have a wax additive in it it must be sealed from the atmosphere in order for it to kick if it doesnt have wax in it spray p v a on it this seals it and allows it to "kick" when dry water will remove the p v a adding acetone to thin the gell cote than adding grip tex to it applying the mix with a foam roller will give a great finish patern to your deck however the chemicals will mess up the roller so make sure you change rollers while working