Regular Hoppes 9 works on metal fouling, both lead (well) and to a lesser extent, copper. Combined with a copper brush on the outside of the barrel, a bit leached into the solution, then combined with one of the more reactive metals in the finish, possibly exposed iron from a poorly kept weapon (previous owner; I read the OP)
Try Hoppes Bench Rest 9 Copper Solvent, and with a fine-wire stainless or simple plastic brush. Shouldn't really need any scrubbing; just a bit rubbing in. Then, let it soak overnight.
If that doesn't work, and assuming your finish is ruined anyway, a weak solution of H2SO4 (try a pool supply place, tell 'em you need just a bit of it), rubbed on exterior surface with a cotton ball (use gloves; the stuff is potent), should strip it. Rinse it off with water, then immediately clean and dry your firearm. Don't use it in the barrel, though, or on any of the internal parts of the firearm, as if left on too long, it pitts.
Can't help you with refinishing it, but another put out some links which looked good. That's my jackleg approach.