I've tried all of the above with minimal success. I bought some rubrail material that had been drenched by the sun for more than a couple seasons. (I know, I know, why would I do that? It was the only one that gave the Martini a more modern look and it fit perfectly!)
I think I may have to actually remove some material to get the yellowing out. Thankfully it's only in a couple of spots. Any suggestions?