I use roloc pads 3" - with course sanding discs , knock down the ridges, block sand to level/ smooth. Look closely for any voids (where trapped air bubbles are) most of them are in fiberglass / gel coat interface layer. You want to open those up and fill with a quality filler - rage has been highly reccommended - Most of the time , I use an epoxy resin thickened with various fillers -( west system for example) micro fibers, micro balloons, etc, - as most of the ridges/ seams are just the parting line / not an actual joint between to separate pieces - most of your work is fairing, not structural. other than filling the voids. Once that is sanded down and you're happy with feel , epoxy prime - (either reg or high build as necessary - block sand, prime as necessary, ( if you've sanded through the primer) - then primer/sealer per paint manufacture specs. What ever fillers you use - allow enough time for shrinkage - full cure and sealed with a good primer coat. You will be going through the gel coat in some areas - it can't be helped. If you go too deep - widen the area and rebuild with fiberglass cloth/ mat , sand back to level, then apply body fillers / skim coat . I think street rod painter has some tips on the other forum - you'll also find a bunch of information on marine repair forums - as the majority of boats are fiberglass - your average body shop deals with steel bodied autos and may not be as well versed in fiberglass prep - and the materials selection - Take your time and when you think you've got it sanded perfectly smooth ( the primer)
give it another go over with the block - (wet sanding)