Start with polish and see how it looks. Avoiding painting can only be a good thing. I had a similar looking problem on my rib and sanded starting with about 240 grit on the nasty bits, working throught 400, 800 etc up to 1200, then farecla Profile 300, followed by farecla Profile Gelcoat Restorer & Wax. It wasn't a perfect finish and took a while, but was a massive improvement on the original finish. I'll try to dig out some photos this evening.