I'm assuming bondo wouldn't work?
Actually, Bondo is what I used because I had some around. It seems to have worked fine. However Formula 27 would have been the better filler to use. If I do the job again, I'll use Formula 27.
The basic order of operations was 1) rough sand, 2) apply glass to build up surface 3) rough sand to fair in edges only and lightly scuff the surface of the glass 4) apply filler to fill in the glass weave and eges 5) fair in filler smooth 6) apply gelcoat 7) sand up to 600 grit wet & dry.
Be sure to wipe with acetone prior to applying the glass, filler, or gelcoat. And don't touch the surfaces with your bare hands after you've done the acetone wipe.