I agree with Lew. I use a sander with either 16 or 24 grit paper and get
real aggressive with it. I would only add you should start by de-waxing the
surface first. It will clean the surface and reduce the clogging of your
sand paper. I use Dupont Y-3919S Prep-Sol solvent. It's available at
automotive paint supply houses. Pettit also makes a fiberglass de-waxer
that's sold in marine stores. Be sure to use a respirator approved for use
with paints.

Rusty O