I assume you are talking about radial whitewall tires since I have had that problem on each set, but never with bias ply. You will notice that on radial tires the WSW is placed on top of the black tire, while bias ply tires are below the surface of the black. I assume that this white is much thicker than the piece paced on top, but I can't verify that. According to a Michelin tire engineer I talked to at the Detroit Auto Show a few years ago: As the tire flexes the chemicals in the tire actually migrate to the outside of the tire. This keeps the tire flexible and last longer. Those chemicals are the ones that cause the browning of the tire. You might make it better right after a cleaning, but it can't be eliminated. That is what the engineer said. I have a set of bias-ply WSW tires for shows but prefer radials for cruising. I talked to Diamond Back tires about this problem. They said they place a barrier layer between the black and the white so that the chemicals do not migrate to the surface in that area. I had them make me a set with 5/8" WSW for my 1966 Mustang. I told them the distances I wanted for the black rubber from the wheel to the white and then the size of the white. I could not be happier. perfectly white all summer long. Expensive yes, but every 6-8 years for tires and not having to worry about browning is worth it. Maybe the reproduction bias ply tire manufacturers like Coker, also place this barrier layer but I don't know. All of my bias ply WSW tires have stayed white.
