Anybody have any experience in doing this? The fiberglass is intact and there is a thick layer of it from prior repairs.
Also, any suggestions of what sanding grit should I use in prepping and finishing?
fiberglass is a very different material than the metal underneath automotive finishes, and gelcoat is very different than paint. I'd be concerned about how well the paint would stick to the fiberglass, and even if it stuck how well it would wear over time.
My view is that since the prep and color matching is 90% of the work with both paint and gelcoat, you should do the finish the "correct" way and use gelcoat. If you post some pictures of the area you need to repair, people on this forum can give you some more specifics about what it would take to do it in gelcoat.