I'm not sure if theres any finish sealing a (stainless steel?) piece. If not, then get some wet/dry metal sandpaper. Start at 180, then to 280, then to 320, then 400. I use kerosene to lubricate the paper. Stop when the sanded area matches the rest of the gun. Gray scotch-brite can be used to blend the area.
If its a blued gun, then just get some cold blue and use either a fine tipped paintbrush or pull the fibers on a q-tip to a point and use it. Brownells has some cold blue as I'm sure your local sporting goods store does as well. Its crappy stuff, but it will be easier than a complete polish and blue job.
Good luck...