From what I've heard (from SkiVisions videos), this works well, but you have to be careful to take the file out of the vinegar with a pair of pliers, not with your fingers touching the file until after you've washed off the vinegar and the reactant on the file surface with water. Cleaning grade vinegar (I think 3%, double normal) is what the SkiVisions founder, Mark Sewell, recommends, to make files sharp again. The vinegar actually etches the metal, I understand.
Files seem to last a long time for me, so I haven't had to try this yet.
Correction: Haven't used this stuff for a while and had an old guy lapse: it's 10% for the cleaning/weeding/etching vinegar, ~5% for the cooking and salad vinegar.
Edited by ski otter - 1/12/16 at 12:42pm