Well I've tried almost everything now, spent 5 hours on 1 wheel yesterday, tried smart wheels, P21s powergel (left this to soak on for 2 hours), normal polish, T-cut, tar remover, halfords foaming cleaner, and black marks are still there.
Its almost as if its ingrained into the wheel. I've never used a clay bar but could this be the answer, bearing in mind i've already tried polishing & T-cut with no joy?
