This is likely to cause some argument and is not a recommendation, it is an observation instead. I often mention my old friend Walter Grass, in his business, he built drillings and doubles, mostly O/U doubles. The barrels were soldered together by a local "rohermacher", and I never met or talked to him. Before rust bluing the barrels, however, Walter had to do the final cleaning of solder from the joints. It is very difficult to see solder residue on polished steel barrels. Walter used hot salt bluing to be sure the residue was gone. Of course, I asked him about this, since it went against everything I had learned. He held to two very important rules. First, this was only used on new barrel bundles, that had never been blued before. The second rule was the time the barrels were in the bath. He insisted that the barrels not stay in the bath one second longer than 14 minutes. The salt bath was also useful to remove solder residue from bolt guns, where sight bases or clawmount bases had been soldered. In these cases the two rules didn't apply because the solder joints were the full width of the bases. He believed mechanically removing the residue with chisels, scrapers, or abrasives stood a good chance of undercutting the thin joint at double gun ribs. This undercutting also can cause the rib to "pop". It has been my observation that when a rib does "pop", it is caused by rust on one side of the joint, and solder is left on the other side i.e. the solder was not eaten away. Walter told me that in the very old days, the entire barrels were tinned to prevent rust under the ribs. This didn't carry through due to the work required to clean it up before bluing. As to the wear of rust bluing compared to salt bluing, in my opinion rust bluing is better. The solution is called the "biter" because it bites the metal. The resulting "rust" leaves microscopic pits. The metal inside the pits is also turned black in the boiling. To wear the color away, the metal has to be worn away past the bottom of the pits. This is my opinion and it won't upset me if someone has a different opinion.
Mike
