Originally Posted by
oldred
A well known trick to refresh a mildly dull file is to soak it in acid but I have found the common method of using dilute Sulfuric acid to be entirely too harsh and while it does work it erodes the teeth rapidly. I have also seen Hydrochloric (AKA Muriatic) acid recommended but it's much worse for a file than Sulfuric acid because in addition to rapid erosion of the teeth it also makes the teeth very brittle and prone to chipping and even breaking. Phosphoric acid, the same stuff in colas and other soft drinks, works well too and will not harm a file since the action is very mild and it won't cause Hydrogen embtittlement like some of the harsher chemicals. This stuff is sold as a rust remover/dissolver at places like Lowes, Home Depot, most hardware stores and even at auto parts stores under brand names such as Ospho, Must-For-Rust, etc just make sure it's Phosphoric acid. I had serious doubts about this until I tried it and it is in fact a pretty amazing process for refreshing (I hesitate to say sharpening) what I would refer to as "utility" files, I still send my more expensive specialty files to Boggs for their abrasion process although I have freshened up a couple of the rifflers with the rust remover solution.
This won't do much for a heavily dulled abused file, Bogg's process can save many of even those however, but for a file that is just normally dull or maybe has been used on a metal that was too hard it has to be tried to be believed! Take the Phosphoric acid based rust remover (this stuff is so mild you don't even need gloves) and pour it into a bottle or a pan that the file will fit into so that it can be submerged completely over the cutting surface, leave it for an hour to an hour and half then remove it, rinse with cold clear water, dry and oil that's all there is to it. The first time I did this I was amazed at the difference it made, I could easily feel the difference just running my fingers along the file teeth but the real proof came when I tried it on a piece of steel, it cut better than new if anything! Like I said it won't revive a ruined file but one that has simply gotten to dull to cut very good will undergo an amazing transformation and that mild Phosphoric acid will not harm it in the least.