"Rust Bluing" and "Fume Bluing" provide the best rust and corrosion resistance as the process continually converts any metal that is capable of rusting into (Fe3O4). Treating with an oiled coating enhances the protection offered by the bluing. This process is also the only process safely used to re-blue vintage shotguns. Many double barreled shotguns are silver brazed together and many of the parts are attached by that method also. The higher temperatures of the other processes as well as their caustic nature can weaken the brazed joints and make the gun hazardous to use.
Rust bluing was developed between hot and cold bluing processes. It was originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes. The process was to coat the gun parts in an acid solution, let the parts rust uniformly, then immerse the parts in boiling water to stabilize the rusting process by removing any remaining residue from the applied acid solution. Then the rust was carded (scrubbed) off, using a carding brush or wheel. A carding brush is a wire brush with very soft, thin (usually about .002 thick) wires. This process is repeated until the desired depth of color is achieved or the metal simply will not color any further. This means they will not rust any further with the application of acid solution. This is one of the reasons Rust and Fume bluing tend to be more rust resistant than any other method. The parts are then oiled and allowed to stand overnight. This process leaves a deep blue/black finish.