i grinder finish everything to about 36 grit very close to the final shape (ya get good with it after twenty years

). Yes you are correct to use minimal fillers. Many times i will get the glasswork close enough that a couple coats of gelcoat will be all thats needed to finalize the surface. I would try and stay away from duraglass it really has no solid place in marine repairs. If you do use it, use it at a minimum. A good marine filler is 3M's premium marine filler, great stuff and works easy (best price at boatfix.com). You should always taper any repair in fiberglass all the way down to filling screw holes, its actually easier with thinner glass. Use a ratio of about 12:1 in other words if its an 1/8th inch thick grind it back about an inch and a quarter on each side. If you were to just fill the gap in the fiberglass without scarfing it back it causes a discontinuity in the surface that, through thermal cycling, will eventually show up as imperfections in the paint followed by cracks.