I have sharpening files often, but I used an other mixture, I like to
be in a hurry. First clean the files with a steel brush an degrease
them in hot sodium carbonate with a brush Rinse them in water, and put
them in a mixture of: 7 part water, 3 parts concentrated sulfuric acid
and 1 part concentrated nitric acid. A few minutes is enough to make
them useful again. After etching put them in the sodium carbonate
again to remove rests of acids.
An other simple way is to etch the files, (after cleaning and
decreasing ) in 25% hydrochloric acid but this takes a day.