Fiberglass questions

You have gotten very good responses to your OP. Just to ad a few things when I was building composite airframes we did a test for the army by leaving an unpainted graphite epoxy structure outside for years unpainted and one that was painted the unpainted one(with no maintenance) show fibers through the surface after 5 years. After 10 years it would leak water through. The painted one held up well. As another data point I had a 1994 HI top van with a FG roof. I was working and I seldom waxed the top. It was badly cracked in the molded radius areas, and crazed in the flat areas. It never leaked because it was about 1/4 in thick so the inter laminar resin and plies prevented that. I have seen many surfaces painted with gelcoat after sun damage and they look fine as long as they are spread on thin not greater than .020 of an inch. Unfortunately when gel coat is brushed or sprayed on by an operator he has to control the thickness by eye. Thicker gel coats look better initially but will crack over time in the suns heat. Resin expands about 5 times more than the FG substrate. You can check the top by coin tapping it, a nice ring to it means the plies are not disbanded, areas that you hear a thud sound are. If you hear a thud sound circle these areas, go inside and see if you have corresponding leaks. The roof is likely 1/4 in thick so you should be able to walk on it and noticed any soft spots also. Hope this helps.