new question. I have a keychain with a charm attached and the metal on the back gets tarnished really fast. I don't use it as a keychain though; I hang it on my purse so it's not often touched. If I put a coat of clear nail polish on, will the polish eventually eat/damage the metal?
Usually, depending on the metal, absolutely not. If it's some sort of weird/rare metal, I'd check with someone who regularly works with it, but if it's just run of the mill nothing terribly expensive keychain charm, then it shouldn't be a problem.
The polish should actually protect the metal, it's the oxygen in the air that's starting to corrode it. The polish will protect against that corrosion by stopping the oxygen and other oxidizing factors getting to the metal to wear it down.
You may find yourself needing to reapply the polish, though, depending on how much wear and tear it gets on a regular basis. If it's constantly rubbing against your purse or knocking into things, the polish could wear off eventually.