Michael
You can use steam, and I have, but as Steven pointed out it has issues.
I've been using a black iron bluing tank for years, the type Brownells sells. So far it's lasted 8+ years.
Rust bluing is easy to touch up, as you can rust blue over many finishes..old rust blue, caustic blue etc. But, one issue you will find when doing rust bluing is the underlying polish.. Something cause that spot to get worn, minor rusting, holster/scabbard/hand wear, bump, scratch etc. Its quite difficult to get that spot to match exactly; even when rust blued, because the difference between the damaged area and the original polish lines.
Generally you need to polish that area out, and feather the polish lines into the original polish lines. I do this quite often on barrels that have been rebored...Polishing off the old caliber marks, then restamp with the new marks...then a touch-up polish & rust blue.
V/R
Mike
