RE: sanding fiberglass
Start as rough as you you need to, to get rid of the speed bumps, and crack the any airbubbles.
Once you get it down to a fairly consistent level, go to the next 100, or 150 grit to clean up the really rough spots. At this point I might consider another coat of resin. Then starting out at 150 I would work my way to 220 or even 300 grit, then spray for primer to look for low spots, or any spots that may have missed.
Then I would probably work the hull with 220 or 300, prime again, then consider a wet sand with 300 before first paint.
If you sand with 220, and spray a light coat of prime, and see horrible rough sanding marks back up one grit, and work the rough spot and feather out the scratches to a nice even consisntency and back to 220 grit.