No need to go through the Gelcoat, just sand thoroughly with 80 then 320, spray the finish in two or three light (reduced) coats wetsand with 600 and compound out to tighten the finish this does'nt add to much weight and works with most topside paints. Your decks could be finished with a foam roller with a bit o " Shark Skin" none skid, use the finest possible grade in some thined finish coat . Your cure time is the most important between coats. "Be Patient".