[Unclenick]

Not just to prevent rust, but when copper was still cheap enough, a thick copper plate acted like body putty to fill scratches, then was buffed smooth for the nickel. I've got an old book on chromium plating shop procedures that describes this.

Today, for guns, electroless nickel seems most reasonable and it can be done at home, but the solutions aren't cheap. You might, however, look at Brownells for the nickel stripper.

For blue, I would forget the cold blue and go to rust bluing. It's pretty easy to do at home for a small handgun sized project and, though slow going, produces far superior appearance and ruggedness to that of any cold blue. Use the site's search engine for threads on rust bluing for instructions.