This one is simple, and it's like magic: If you have Taylor Made fenders, the problem is that the surface of the fender itself becomes compromised and won't clean, so the real absolute answer is Acetone (fingernail polish remover). Just go to a hardware store and get a quart of acetone, rubber gloves and paper towels. The acetone literally melts away a very small outer surface of the fender, leaving it pure white and gleaming again. Never use water when cleaning them with acetone. Just get a paper towel wet with acetone and wipe the surface and you'll be amazed to see it completely come clean. Turn the paper towel as soon as it becomes dirty and throw it away before it becomes dry.
Be careful not to wipe the top black part and then the bottom area because the black will come off too and can mix with the white or other color, but if you do, another clean acetone soaked paper towel will take that off.
This really works! I just did mine yesterday since we have gone through the last of the locks until the Hudson River in New York. They're gleaming white.
And no, it doesn't take off enough to make them weak.
Doug Shuman
BTW - this cleans your shore power cords too.