Only approach that will work is to sand off all cracked gel-coat. Maybe use chemical stripper on the paint on top of it first. 60 grit paper works fast-if you do apply high-build epoxy after[a good idea] scratches will easily be filled. Keeping all surfaces fair can be tough-it helps to have a selection of hard sanding blocks with profiles that fit your boats molded radius -easy to make from cut-offs of plywood or foam core.