Some 30+ years ago I used to Acid Fume Blueing on rifles and pistols, (never on soldered shotgun barrels). All boiling of the barrels was done in deionised water. After the last coating of rust was removed, the barrels were boiled once more in a fresh tank of deionised water, the barrels were removed and quickly dried off with a clean piece of muslin and then a coating of beeswax was applied by rubbing the stick of beeswax over the hot metal, and the heat of the barrels melted it so it filled the microscopic pits left behind by the rust blueing. When cold the waxed barrels were polished off, again using very clean cloths.
I mentioned this process to a friend who is a gunsmith in California and he used this process for some time and found it very satisfactory indeed. He now used the fume blueing method and also another rust blueing formulae by A.O.Zischang (slightly modified) which can be found in Angiers book on Blueing and Browning Firearms. He still waxes the barrels after using both blueing processes. It provided a beautiful finish.
Harry