When I was somewhat new to collecting and owning firearms I came to learn the people considered a reblued gun ..... ruined! This to me did not make sense, and still does not. I believe that this mentality is born from a belief that any reblue will cause the firearm to look different from what it originally did. Maybe this was because gunsmiths were caustic bluing antique firearms?.... I have no idea?
Based on the very nature of what a blue finish is, as long as roughly the same process is followed, the gun will look the same regardless of what chemical is used, or how clean your water is....etc. Conversely a gun that was originally rust blued will not look correct if it is caustic blued or some other finish is applied.
There are quite a few myths in the industry about bluing. I have formulated an experiment to help dispel a few.
As mentioned in my article "Rust Bluing to Winchester Recipe?" I conclude that the colour of the blue finish is a function of the material itself and the fundamental colour of Magnetite not the chemical used. To prove this I came up with an experiment whereby I compare the blue finish of a barrel blued on one half using my homemade blue solution and the other half with regular old salt water I collected from the beach.
Above is a picture of a single shot shotgun barrel I had lying around. I stripped the old blue off and polished the barrel using 400 grit wet and dry sand paper like I always use. I placed a strip of tape around the barrel. The chamber end of the barrel was blued using my homemade blue solution, the muzzle end was blued using salt water.
In the above images, it appeared that the barrel was going to look different. My whole understanding of rust bluing must be so wrong.... Not quite. This image simply shows that the rust blue solution is more corrosive than salt water. In the 2nd image it can be seen that the barrel is getting closer in colour. This highlights the importance of multiple applications. As the subsequent applications are applied, the part being blued rusts more when the blue is thinner/less built up and thus "catches up" in colour.
From the images above it can be seen that there is no difference between the side of the barrel where my homemade rust blue solution was used and salt water. Further proof that the colour of the blue is a function of the material and the fundamental colour of Magnetite.