Pitting on the external surfaces of a firearm is often thought of as impossible to correct. Extremely light pitting can be removed using an abrasive like sand paper, then reblued. If the pitting is a little more substantial there are some characteristics of rust bluing that can be used to correct this annoying and unsightly problem.
Firstly let’s consider what causes pitting. Pitting in steel is usually formed as an advanced stage of corrosion. Rust in the form of red oxide is not geometrically stable, it will expand, flake and adjust the dimensions of the original steel component. If this is the case, once the red oxide is removed, pitting will remain. The pitting is effectively a local area that is a crevice compared to the surrounding area. No matter how much you polish this area, the pit will remain.
To fix this pit, either the surrounding area needs to be lowered to the depth of the pit, or the pit needs to be raised to match the height of the surrounding area. With regards to firearms, lowering the depth of the surrounding area is not really feasible. The wall thickness on a barrel is important and reducing it would be quite risky, not to mention hard work. Raising the pit however is not impossible when you consider that rust bluing actually adds material.
The thickness of rust bluing is typically between 2 and 3 microns (µm). Using a process of rust bluing,sanding and repeating the pit can be filled as shown below:
Now you’re thinking…. Oh wow I have that original Winchester 1873 that has pitting all over, I will just get this process done and next thing I will have a brand new looking firearm. Well, we have to be realistic about the ability to fill pits. Consider that a hair is approximately 50µm in diameter and that bluing adds only 2µm at a time. Assume that it takes 5 applications of rust bluing to build it up to 2µm, it would take a total of 125 rust blue applications to fill a pit the same diameter as a human hair!
With this knowledge, I would like to stress the importance of light in the term light pitting.