I quit scrubbing my whitewalls with a stiff brush and switched to a Mr. Clean sponge, night and day
Last spring I was scrubbing the whitewalls on my 1966 Beetle with a stiff nylon brush and Wesley's Bleche-Wite, and I kept getting these gray streaks no matter how hard I went at it. A guy at the Lebanon, Ohio swap meet told me to skip the brush and use a plain Mr. Clean Magic Eraser with warm soapy water instead. Two passes took off 12 years of baked-on grime in about 10 minutes, and the rubber didn't get chewed up like it did with the brush. Has anyone tried the eraser on older, cracked whitewalls, or does it tear them up?
Wesley's Bleche-Wite is actually made for tires, but the gray streaks are usually the whitewall's top layer coming off, not dirt. A Magic Eraser is just fine grit sandpaper, so it works by shaving off that layer, which is why it looks great at first. On old cracked whitewalls it will grab the edges of the cracks and pull little bits of rubber loose, so go light and keep the sponge wet.