How do you remove black marks from action figures?

I have around 20 WWF action figures from 1997 onwards that I would like to clean up and sell. However, I have come across a problem - some have black spots/lines on them that are very difficult to come off. So far, I have tried denture tablets in a warm water solution, soapy warm water and Methylated spirits, none of which work. What else is there left to try?