I'd suggest that prep your fiberglass as much as you can--as if you were going to paint tomorrow.
Sand a bunch. Sand until you think you're ready for paint with no pin holes or imperfections. Then spray some sandable filler-primer. You'll be amazed how the imperfections show up. At this point you'll probably want to use some filler before you sand some more. I've been using some stuff called
Superfil from Aircraft Spruce. I used this based on advice from a guy who built a VERY nice Lancair.
After you've filled all the little pinholes and imperfections with Superfil, sanded again with ~320--400 grit sandpaper, then prime again. Repeat the process until you see no more imperfections. When you see no more imperfections, spray one more coat of primer and leave it alone until you're ready to paint.
As for filler/primer, I'd suggest an Evercoat brand product called
Uro Fill.
Another little thing I learned from hanging out at my brother-in-law's body shop is how cool "Guide Coat" is for sanding fiberglass. All guide coat is is a light splattered dusting of a contrasting spray paint on the grey-primed fiberglass part. As you sand, the splattered dots of the guide coat make it very obvious where the high and low spots are in the fiberglass.
SEM makes an actual guide coat product, but I would think just a spritzing with a contrasting primer color from any old rattle can would work, too. I wouldn't use a regular paint because I think it would be too thick and might not sand as well.
It's amazing all the little tricks the paint shop guys have to make this stuff look good!
Best regards,
Rod Woodard
Loveland, Colorado (KFNL)
Midget Mustang @ the paint shop
RV-8 #81964 holding at the wings