Before “sharpening” on the left, after on the right. I guess this wasn’t such a hot idea!
There is a simple shop method for sharpening files that’s mentioned in more or less detail all over the web. It’s done by dipping the files in acid for an hour or more, which supposedly etches away metal, sharpening the edge. However, as the process moves on, heat is generated, and that heat concentrates at the thinnest places in the metal–the sharp edges–and speeds up the process there. The result is as on the right in this microphoto: a file which feels much sharper because the surface has become greatly roughened. But look at what’s happened to the cutting edges: they’ve become blurred and dulled! The fresh, grainy roughness made that file feel like it was really going to hog wood, but in reality it mostly made a lot of grinding sounds, producing very fine dust, and hardly cutting at all.
There’s one shop, Boggs Tool and File Sharpening Company, that really does the job right, though. I sent them a batch of files many years ago, and they came back sharper than new, as Mr. Boggs had promised me. The improvement was genuinely amazing. I suspect he uses a similar process, but adds a step to concentrate the etching where it will make the edges sharper rather than duller. I may have figured out what that step might be, but I think that the next time I want an exotic $30 file sharpened, I’ll just let the expert do it right, rather than destroy it all on my own.