I've come across similar before--seemingly reaction to environmental agents and glue residue (where glue has 'masked' portions of the coin's surfaces). In my experience, acetone has had little if any effect with this particular instance. Now if the glue/adhesive was still caked on the coin, that would be a different story. Anyway, give acetone a try. Can't hurt.
Edited by ijn1944
11/19/2020 10:59 am