Try Dri-wash'n'guard tire and wheel cleaner. Its made by Enviro-tech out of vegas. My sons jet-ski had mold built up on his seats. Nothing would get it off. I was a dealer then and hit a spot with the tire and wheel cleaner. It came right off.
I was a dealer,but quit due to lack of sales and dealer charge.
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95 Dodge 2500 Luverne grill guard,headache rack,running boards and Amzoiled. Soon to be mildly bombed. 84 Dodge d-150, 318 Hooker headers,Edelbrok intake,mallory ignition,Carter Afb,Accel coil,and Custom dual exhaust. Boat,fifthwheel,motorcycles,and shop. 72000 as of 3/1/01,not even broke in yet. Old Dodge 126000 miles and running better than new.