Here's a few pictures of what can be done with sprayable Polyester primer known as Slick Sand, Feather fill, Z-chrome etc....

This was a fiberglass coin operated ride I painted a few years ago. It had been painted between 18-20 times over the years, and removing all that paint would have been too much work. I sanded the paint with 100 grit just to try and get a half decent base to apply the Polyester primer. This primer is basically a thinned out body filler that uses fiberglass hardener (MEK). The pot life is about 30 minutes so it gives you plenty of working time. I use a gelcoat gun with a 2.5 tip to apply it.

Once cured, I sand it with 120, then 320. Since there's no bare metal, I put a heavy coat of High build Urethane primer directly over the polyester primer. I usually let Urethane primer sit for a week, but this little car was sanded the next day with 500grit dry. The top coat is Base/Clear.