Have you tried Westleys ( spelling may be wrong ) tire white wall bleach and cleaner? Also Castrol super cleaner, and Simple Green full strength wok well some times may have to use some fine scotch brite with it, or a stiff nylon brush. all this stuff work really good. If you have no luck cleaning them up, my vote for new tires are the Diamond Back classic tires. Have them on my '51 merc. like them a lot, no problems.---Jim