I am no expert at this, but I'm beginning to think one doesn't have to be. I've just now gotten around to having some experiences using files that I've soaked.
Long story short, I'll never discard a file again without giving it at least one or two acidic baths.
I never got around to sending off files to a refurbishing service, but did recently find that an acid-bath can freshen up most files. I started with old, terrible files and they started looking good, so I soaked a bunch more. But having plenty of files I'd not really give an serious workouts to these files, but they all usually felt really fresh and sharp to my hands when cleaned from their bath.
I use household vinegar and let them go 2 nights minimum. Also this makes a great bunch of iron acetate "stain"-if you'd like to try such (but that's another topic).
What really surprised me were these two extremes. First was a "found on the ground" type bastard with serious scale and pitting (looked hopeless). Needing a good long file to fix an ax the other day I grabbed the first 12-incher I saw and started working the bevel. As the bevel was dirty and nicked up I didn't notice it right away, as the file was biting into the steel just fine, but something didn't feel right. I then flipped the ax and started on the other bevel and got the same "not proper" feedback. THEN I looked at the file as I knew I was into clean metal on the work.
The file was the pitted one. I was feeling the pits of course, but the remaining teeth were cutting the dog out of the steel. WOW. So that's a truck-file for sure. It'll be great for field/coarse work and was totally useless before.
Second file didn't feel right when I cleaned it from the soak. So I resoaked it. Still didn't feel right. Put it to metal (that same axe) and it won't bite-and it's the FIRST one I've gotten no renewed sharpness from.
It has a name on it. Naked eyes see only an "M". 4x microscope reveals the "REST OF THE STORY"--which is "India". So that one goes into the bin and I'll be careful not to purchase any similar again.
Vinegar, file (of good stock), 2-nights, scrub off the nastiness and lightly oil. See what you think. Cheers!