I had the same question for CLC. They recommended at least a light fill coat on the bottom before painting. I didn’t apply a third coat for uniform finish though since it is getting painted instead of varnish (and no one will ever see the bottom!) I did apply at least two more coats on the side of the camper where the fiberglass overlaps the edges.
I made choice to paint while it was inverted to save having to flip it again later. I start varnishing on Thursday. Plan is just to tape over the paint line with high quality painter’s tape and be careful with the varnish.