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I have used rust converter to clean coins . It has a lot of phosphoric acid in it . This will remove the crap and not attack the metal . Try it but keep an eye on them . I would not advise to leave them in for a long time . If you think it is acting too fast , you can dilute it with some water .
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Phosphoric acid is used in some etch primers.
It can/will etch metal.
A guy l worked with years ago said he’d show me how to clean a bicycle chain.
He threw it in a bucket of phosphoric acid, the next day it was a black sludge in the bottom
Chain must have been made in one of those large over seas countries ............
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