better living through chemistry...

my only advice is acetone. it doesn't react with metal--it's been suggested to my by the nabobs at the grading service as a way to get surface off, everything from pvc to generic dirt. make sure that you get pure acetone (walmart or a local hardware store will carry it), dip the coin in for about 10 seconds, rinse with warm water, and then dry on a soft cloth. i did that and then sent my coins in and presto, slabbed with no problems (by the nice folks at ngc).

hope this helps.