Many answers to this one:
All cold blues are acid based to etch the metal surface and grow the black selenium compound crystals. The etching activates the metal, so you have to completely neutralize the acid, then apply something to either occupy the remaining activated surface spots that are microscopically still exposed to the outside world, or react them with something.
The exceptions to the above are those cold blues based exclusively on phosphoric acid (no nitric or hydrochloric acid on the label). These blues will actually perform a kind of cold parkerizing. It is really a dark gray finish, and not as black as the other blues, but much more rugged and less prone to after-rust. Two examples are Van's Gun Blue and Brownells Oxpho-blue.
In all cases, after using, neutralize the acid. Spray with Formula 409 or Fantastik. These cleaning agents are so strongly alkaline they will etch aluminum. (I've heard Simple Green will, too, but haven't tried it yet.) Strongly alkaline cleaners will both wet and penetrate and neutralize acid.
Next you want to rinse well. Rinse with hot water. A hot barrel not only dries quickly, exposed metal tends to form ferrous oxide and ferrous ferric oxide (magnetite) which are blue/black in color and will protect the metal.
Next apply a rust inhibitor that bonds to the metal surface. This will go to activated sites first. Birchwood Casey Sheath and LPS-2 or LPS-3 are examples of this. WD-40 will displace water, but for long-term rust resistance it is inferior to the others I mentioned.
A third option, if the surface rust is fine, just boil the barrel in distilled or deionized water (what comes through a water softener is usually fine). It will turn black. This is the principle rust bluing is based on. You can then wipe most of it off, but what sticks will match the bluing. You still need to use a good rust inhibitor with this.
A forth option: Use the darker blue and then, after rinsing with hot water, apply the less dark, but more protective phosphate bluing as a secondary surfacer to react with activated points remaining on the surface.
A fifth option: A new product called
Blue Wonder has been introduced. They make an excellent barrel cleaner and also make a cold bluing. I bought some of this and made a test on a piece of steel. I usually do this with any new cold blue I am trying. Its instructions ask you to apply some hair dryer level heat. It reacts very slowly at room temperature, which is good because it means the acids in it aren't very strong and don't etch excessively. The resulting finish, like the phosphate blues I mentioned earlier, does not appear to tend to surface rust easily. It is also darker in appearance and more mirror-like (because it doesn't etch much) than any of the other finishes. I can't tell if it is genuinely darker or if the glossier surface just makes it look that way? It may be the winner, but I haven't got enough experience with it yet to be 100% sure of what it is capable of.
Nick