What rust blue do you recommend, also Ken you stated a weaker solution?
Do you delute with distilled water? Applying multiplecoats before steaming, then card off with fine steel wool?
First, a few points/questions. What are we "touching up"? Is it a small piece like a trigger guard? Is it a barrel? If a barrel, is it fluid steel or Damascus?
I make my own solutions, but I'm sure others here buy theirs, so I'm sure they can offer recommendations. Many are Aqua Regia type solutions that work fine, with care.
For tubes, the judgement call comes when trying to match dilution with existing coloring, as it's better to start weaker than stronger, as it's harder to blend a darker spot into lighter background. Application needs to be fairly precise, with little bleed-over which would darken lighter backgrounds. I use pointed swabs (make-up applicators) and small artists brushes. Sometimes I allow it to rust a few hours, other times I steam fairly quickly, all depending on darkness of background color. One application, then a steaming/carding. It's better to err on the side of lightness, as you can always repeat the process with stronger solution. If Damascus, you have to periodically have a light vinegar spot-etch, a very tricky business. At the end it's a process of blending, not only with Steel Wool, but with pumice paste on the end of a swab if necessary. Chalk paste makes a good final polish, but at that point it's a judgement call.
Cold Blue, in my experience, (which may differ from others) doesn't work on Damascus or Twist because you can't etch it to bring out contrast. If anyone has been successful, please post your results/procedures.
Yes, distilled or filtered rain water is used for dilution.
Regards
Ken