FWIW, I copied this from another forum, dunno how good it works or what kind of media to use as I don't have a "grit blast cabinet".
If you have a grit blast cabinet or know someone who has; give the files a quick grit blast and they will come up good as new.
Would a grit blast cabinet use corn meal?
Ken's method is how Ford sharpened their files. They claimed that after the first re-sharpening using this sandblast method, the files held their edges 10% longer than new ones. Maybe the heat from the friction had a slight hardening effect.
A soak in mild phosphoric acid will also re sharpen them and leave a rust resistant coating. KGB
Steve, I prefer my grits fresh, I'll pass on the grits after they've been used in your blasting cabinet!
Nothing better than good homemade grits, and nothing worse than a dull file!
I have done mine by soaking in Acetic (White Vinegar) acid for about 48 hours and then wash in water and oiled real good.
They get sharp but seem to "pin" a little faster than when new.
Well Les, I guess the grits with the white vinegar washed in water might be to your liking but I'm concerned a little bit that you don't mention what type of oil you're using.