Hi Steve:
Communication takes effort. I think I've answered a question, and then I realize I've left out half the story.
Yes, I sometimes start with 100 grit paper, and finish
with 220. Use your judgement. The 100 grit is good
for removing drips. So is a sharp scraper. But NO, don't even think about sanding enough to obscure the pebbled surface (of your raw fiberglass finish).
The main purpose of the 220 is to make a good bonding
surface for the next coat. Subsequent coats ( of hot,
thin resin ) will fill the valleys (of the pebbled
surface) more than the peaks. Sanding between coats
also enhances this "valley filling" procedure.
When preparing for a paint surface you can mix the resin
with microballoons--which are easily sandable. You can
prepare for paint in one coat. For a clear finish you
may need to coat (with clear resin) and re-sand 2,3, or
four times. Think about body shops, that put 6 coats of
color and 6 coats of clear on top of a hot rod.
Special finishes take elbow grease ++