This suggestion is ONLY applicable to zinc coins.
Have you ever encountered a basal zinc coin? Apparently so corroded that you cannot read the inscription? Before you throw it away, try soaking the zinc coin in vinegar for an hour and a half or two hours, before washing in cold water and rubbing gently with your fingers under the water flow and then rubbing briskly on a piece of cloth.
You may be surprised as the results!
I hope to produce a Numisdoc article on restoring coins in the future, first though I need to obtain some more badly corroded and/or dirty examples to work with!