For some rust spots, I have sucessfully used a single edge razor blade. First coat the spotted area with light oil. Let it soak a bit. Then at each spot , make sure it is well oiled, then take the razor blade and "shave" off the corrosion, keeping the flat side of the blade as parallel to the barrel surface as you can and still get a "shaving cut". On tougher spots one might do the process in two or more applications. The lubricant allows the blade to slide on the bluing without harming it.

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