I would try a Dremel and wire brush, but only on a very low speed setting, and I'd test it first.
I believe that there's a plastic wire brush for the Dremel.
If that's not an option for you, I'd stick to the razor blade. You could try a toothbrush, but I doubt it would do much. Maybe... if you trimmed the bristles, so that they're stiffer, you might get lucky.
