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A toothbrush is too hard on the nickel. Buy a ½ or 1-inch boar chip brush and cut down the bristles to about ½ inch and use that with dawn dish soap, follow up with alcohol and another paint brush.
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Yup, depends. Some soap can turn to near cement and will gum up the working action. The problem is the Nickle is very thin, so if you use harsh cleaners you can strip the Nickle.

I have...