You may have already considered it, but I would try "Starbrite Fiberglass Color Restorer." I've used it for several light applications on hulls and top decks like this and am very pleased with the results. Google it, $12.99 @ wholesale marine. It'll take out discoloration & light oxidation. Don't need a buffer & it's not very intrusive. Just apply a small amount with a rag and really work a small area at a time, then rub off with clean dry cloth. Even though it has PTEF, I like to apply marine polish on it afterwards.