nicotine remover

Does anyone know what I should use to remove what looks like nicotine
on the metal parts on a banjo uke I am restoring? I have some parts soaking in dove dish soap and some in paint thinner , so far with no luck. I'm thinking of trying lacquer thinner. All the metal hardware has a brownish film on it.
It's not my nicotine, I don't smoke cigarettes. Now I'm thinking maybe some white vinegar will do the trick. Any ideas? Thanks.

If you can isolate the metal parts try Goo Gone

Are you brushing in addition to using these liquids? Just soaking isn't likely to help... I'd try some metal polish and a tooth brush.

The thing that seems to be working best is the white vinegar. I've taken all the metal parts off and they are soaking. Ill brush them with a tooth brush and then buff them with simichrome. I think the white vinegar will do the trick. Thanks