The first is a quite complete and scientific look at cleaning older silver and silver-copper alloys
https://www.jcms-journal.com/articles/1 ... s.1021204/
The second is a not very scientific look on what to expect cleaning copper-nickel
https://world-collect.com/article/chist ... vikh-monet
(Note whizzing coins and recoating them by electrolysis is quite frowned on by the numismatic community ... in short, unless the patina is very fine, with cupro-nickle you are likely to end up with a slightly more presentable coin but that is obviously cleaned, with a grainy surface, and erroded features... fortunately most copper-nickel is more recent, but usually best to leave as is with just a surface clean for dirt )
Brass, bronze and copper are their own world , the first two particularly awkward . If I find a complete guide to any of those I will add it here.