Thank you for the comments & suggestions. I would never do anything much to a collectable coin - those that are listed are mostly exactly as they arrived with me. But I get post 1948 cupronickel coins which are black. They are difficult to spend or exchange and I need to clean them. Coke is making a tiny impression but not much. I'll work through the other suggestions and report back.
However, I have had a bright idea - I can use the discoloured coins for parking meters! They are proper coins and worth the same as they ever were so no-one is being cheated.