Use ordinary vinegar to remove bluing from a gun or any steel object. How to prepare an M1911 pistol in preparation for parkerizing.
If it is steel, does it looks like it might be painted black, or does it looks more like the "blued steel" on a gun barrel? The rust can probably be removed with very fine ("0000") steel wool ⇦ on eBay or Amazon [affil links] , but even this can scratch off the blackening....
Polish the parts to be blued. Again, steel wool or 600 to 1200-grit sandpaper will work to remove any loose rust, scarring, or pitting from the gun metal. If you plan to remove the gun's finish from the factory before bluing, some recommend using a phosphoric acid-based automotive ru...