I have been struggling with this for a long time for the BF Goodrich radials. I have tried:
soap pads, lacquer thinner, Bleach White tire spray, sand paper, other things I can't remember. None of it worked well.
What did work was a white Sharpie paint pen, got it at Home Depot. There are other paint pens, but they don't work well, so be careful. I got this idea on YouTube.