There is a world renowned rifle maker in Cache Valley Utah that tested different types of bluing (different rust blue formulas, hot blue formulas, I believe even carbonia bluing). Same steel type, etc. put the pieces of steel in a tumbler, sprayed them with mild salt solution, etc. Basically torture tested the finishes. The hot salt bluing in which the salts were changed frequently or new came out on top every time. Guess which finish is used on his $22k custom rifles?
Like what was said earlier, sweat and natural body oils found on the hands, etc are terrible for any type of bluing.
