I have used acetone myself and have sent the coins to get graded after without a problem but be sure its 100% acetone not finger nail polish remover with acetone in it. You can buy 100% acetone at just about any CVS/Eckerds or the like stores for around 3 bucks. Just rinse them off good with warm water after you have taken a cotton swab or q-tip to massage the coin, never rub hard with a rag or anything like that because then you may get the dreaded "body baged" coin back from the grader. It is always better to just leave the coin alone and not clean it but if it is just layered with gunk like some I have had that is the process I have used and never had a problem with getting them graded at all