A good thick latex paint
with some moldicide is a good choice for the undercoats. It lays up thick, doesn't smell bad, and smoothes out the roughness of the fiberglass.
Then when it is all dry you can top coat it with enamel or something else if you like.
What I did was to purchase some of those blue foam rolled up hiking mattresses from Wal-Mart. They are about 2ft by 6ft and cost around 6 bucks each. After all the painting
, I stuck them on with "Liquid Nails" adhesive using small blobs on about four inch centers around the edges and relaxed that a bit in the centers.
A coat of latex paint changes the blue to white, in my case, and stablizes the surface somewhat.
This is not an original idea of mine. I recall coming upon it while lurking around this site, or maybe a boating site a year or so ago.