That's a tough one....
Hard rubber is hard to correct and make look new and this component of a boat is designed to take impact and abuse to protect the rest of the boat.
Believe it or not I've had a little luck with the Mr. Clean Magic Eraser and we sell something like it here,
SONAX Dirt Eraser - 2 Pack
Besides that, using an all purpose cleaner and either a bug sponge or some version of Scotchbrite pad.
Lake Country Exhaust Polishing Sponge - 2 Pack
