Also, if you have some surface rust, I have found that if you take a copper penny (made before 1982) and scrape the rust with the edge of the penny, it will scrape off rust well without even damaging bluing. Put oil on the rust before scraping. It's a neat little trick that I have used for years. Scrape carefully with oil and see what happens. Helps a lot for surface rust but cannot fill in holes if pitted, of course.