The fiberglass gel coat is likely toast.
Do not try and use a wax or a polish at this point as it will get into the cracks and repel any future coating
Look into this paint.
http://www.amazon.com/Rust-Oleum-206999-Marine-Topside-1-Quart/dp/B000BZTJT2
Proper prep is required. Wipe with acetone, then sand with 150 grit until no shiny spots exist anywhere. Sanding without first removing waxy or oil deposits forces them into cracks which then repels paint and/or reduces adhesion.
Do not allow acetone to puddle anywhere. Put on rag and wipe quickly, flipping rag often as it colors with the gel coat
Apply paint with a smooth foam roller on a warmish day not in direct sunlight. Wait the 22 hours between coats. Do not apply too thick. Expect to need 3 coats and nearly a gallon to do the whole roof, perhaps 4 if a dark color.
It is possible to reapply new gel coat but you would spend significantly more.
http://www.heartlandboating.com/August-2013/Dolphinite-GelMatch-All-In-One-Gelcoat-Aerosol-Can/
My whole Van is painted with the Rustoleum yacht enamel in the link and it is far far better than regular rustoleum enamel paint, which is pretty poor and goes chalky quickly.
The Yacht enamel is scratch resistant, has very good adhesion, and can be waxed, which helps keep it cleaner longer
If you've got a bit more coin to spend on better paint, check out Interlux Yacht enamel.
http://www.yachtpaint.com/usa/diy/products/finishes/premium-yacht-enamel.aspx
My biggest regret was using the Hy-tech insulating paint additive. It makes the surface very rough. I applied 4 more coats without the additive, after the 2 coats with the additive, and it is still textured and difficult to keep clean. I even sanded after the Third coat not caring if I damaged the insulating portions of the additive layer.
I've no idea if the insulating additive does anything either. I doubt it. Look how gritty it is:
This is the hook for my rear door ladder:
The theory is that these little gritty spheres are filled with a vaccuum and they magically float to the top of a paint layer and create this super insulative layer. In reality they are nowhere near numerous enough and the paint would never be thick enough to allow a larger amount to magically float to the top of the paint and do what it says.
Use it if you want an anti slip additive that would be impossible to keep clean, Don't bother if for insulative purposes.
Have a look in your roof gutters. You might be attending to some rust, as it seems these roof gutters were designed to absorb moisture and rust profusely. If so, scrape the loose stuff off, treat with Ospho, and paint to slow it down.
The trim ring just over the roof gutter will likely hinder access to the roof gutters, and this job, if badly rusted, is not rewarding.
Oh yeah, if you use 3.5 cans, then before storing the paint, fill the airspace in the can with propane and press the lid closed tightly. I just opened a can I closed 18 months ago, for some touching up, and there was no skin on top, whereas the can I did not do this with was unusable after 3 months.
My roof paint will be four years old in March and is still in good condition, with waxing once a year, if that.
My Gelcoat was dark gray originally , and badly cracked, as the van spent 13 years in Florida before my acquisition of it, and the roof was painted white within 6 months with Interlux, but I put it on too thick in too cool of weather and it stayed soft.
