The hot bluing method does take special equipment and is hazardous as the molten salts are caustic and are heated far hotter than boiling water. You also have to invest in and learn to use buffing equipment properly to get a deep blue-black glossy result. It is probably the most durable kind of blue, but it's easy to get a couple thousand dollars tied up in all the tanks and burners plus the gas plumbing and etcetera. Fine for a small business, but for the individual that is a bit daunting.
It can be done for a lot less if you can weld your own tanks up and want to spend some time salvaging old furnace burners and the like. You can also heat the salts electrically, if you don't mind the greater cost per BTU, but you'll still have $400 tied up in the first purchase of minimum quantities of the chemicals and degreasers and cleaners and polishing compounds.
The rust blue method is much more appealing to the home user, but the result is a blue with a slightly satin sheen rather than gloss. It is the standard for custom shotguns. The old H&R revolvers had a similar color blue. It is far more even and durable than cold blues, which are difficult to get good whole-gun results with, but not as durable as the hot salt blues. The black blues from hot salts are achieved by adding nickel compounds and other constituents not present in the ferric-ferrous oxide blue. I know of no way to add those in by rust bluing.
There are several approaches to rust bluing these days. The old fashioned method is to build a steam cabinet as described in Howe, which you can read free here (Vol.1
). You remove the old blue (Iosso has a good stripping solution
for this), do any buffing or polishing of scratches you intend to, then degrease the metal. Put the parts to be blued in the steam box together with a watch glass that has a few drops of nitric acid in it, then run the boiler to drive steam in and surface rust it over a period of hours. After the rust forms you remove the work piece and boil it water. Distilled is best to avoid leaving water marks. After about 15 minutes of boiling the red surface rust has turned black. You then card it, which means rubbing off any loose and poorly adhered particles of the blackened rust. This is usually done by using a soft fine stainless wire wheel on a buffer, but degreased steel wool may be used to rub it off by hand.
What you get at that point is a kind of salt and pepper look, as the blue has not adhered everywhere. So, you repeat the process. The well-adhered blue does not re-rust, so only the exposed steel repeats. After about half a dozen such treatments you get a pretty dark and good blue.
In place of the steam box, you can use the Pilkington Blue solution sold by Brownells to get the surface rust to form in air. That takes overnight for each pass through the process. I understand the Mark Lee solution works a little differently and is faster. I haven't tried it, but have used the Pilkington solution a number of times and, though expensive at $30 a bottle, I can say that it works well.
You'll find you will do best if you can remove a barrel from its receiver before bluing to avoid water or chemicals working their way into and starting corrosion in the threads. A lot of places just throw guns into a finishing tank without doing that, but you also see corrosion showing up later in many of them. If you don't take that precaution, then you want to submerge the receiver end in water displacing oil for several days. I've heard WD-40 is the cheap way to go for this, but it leaves such a sticky rust inhibiting surface when it dries, so I've avoided it in favor of the type Brownells sells. If you do use WD-40, you will probably want to wipe it all off with mineral spirits and apply a gun oil.