hey John,gel coat must be sanded for adhesion reasons.you don't want to spray over any shiny surfaces,what your spraying won't stick.breaking the shine to a dull finish is what your looking to spray over.when sanding gel coat,the dust will be the color of the gelcoat.most fiberglass boats that you see are white gel coat.it has a choppy look to it.the gel coat is sprayed on parts because the 'glass is rough and not a very good surface to apply sealer/paint over.the sanding of the gel coat comes down to personal pefrence.i'd sand it with 400-500 grit and be sure to seal it before painting.but,you could sand it and not seal it,however it would take more coats of paint to do so but i'm not fimilar with your painting situation.everyone has diffrent ways of doing things,i'm sure someone will post an awnser or two.any ?'s just ask.good luck.
