I start with 1500 or 2000 and if that isnt working to restore the gel to FLAT (as seen with a pocket microscope) than I increase the grit .... an finish sanding back at 1500/2000. Once 'flat', the I use polishing compounds -- Super Duty (maybe) then Finese-it, then Perfect-it. Once 'flat' you really dont need much 'compounding' to bring the gelcoat back to showroom new'

If however when you look through the pocket microscope and you see what looks like a pond that has dried out with zillions of cracks and 'wafers' ..... do a trial of sanding down through them and 'decide' to then either do a re-gelcoat or paint the hull ... 'alligatoring of gelcoat' is a good signal that the gelcoat is 'over' and any attempt at restoration will be a waste of time.