What a great thread! I think we've all had this problem at some point or other. Black Wire corrosion, eh? Well I never.
The oxalic acid idea sounds the best to me. I normally struggle with needle files and rolled up wet-and-dry abrasive paper - both of which have already been suggested, and which often leave a lot of contaminants of their own behind. I remember in the early eighties my father used to make electrical circuit boards by using an acid to etch a thin copper sheet bonded to a plastic sheet. I've noticed that the acid layer of black mud at the bottom of stagnant water also removes copper corrosion. In that case the acid is sulphuric acid (a biproduct of the sulphate reducing bacteria), so battery electrolyte should work too. Failing that, phosphoric acid definitely works on copper (and steel). I think you should be able to source some dilute phosphoric acid from gun shops as I believe it's what they use for gun barrel blue. Perhaps the salicylic acid used to treat veruccas would also work, and comes in handy tubes of gel.
One obvious thing to watch for would be traces of grease; those circuit boards my father made used wax to preserve the parts he didn't want to etch. Oil or grease would do the same.
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