I know about a year or so ago there was a post regarding where to send Machinists Files out to be sharpened. While the cost was quit reasonable, here is a short video I stumbled upon (accidentally) that shows a very similar method that can be done in your back yard or garage - probably too smelly and dangerous to do in your kitchen. I doubt it would cost more than $10 bucks for everything you need and most of us probably have all that is needed right at home.

While I have not personally tried this method yet, I fully intend to next time I have a few dull ones. Please note the cheap copper cleaning tool he makes out of a piece of copper pipe.

It works,,garage mechanics used to use old battery acid (sulfuric) to sharpen files.
I've tried both methods,,this and sending a box full to Boggs tool in CA.

No comparison between the results IMHO,
The old files I get back from Boggs are like new and the cost is minimal, anywhere from a few cents to a couple of dollars for a the big wood rasps.

I will say though that if I didn't need them to make a living with and do precision hand work, I'd probably not bother and just do the acid etch sharpening. It will bring some life back to them.
It all depends on what your need from them.

I can believe that Boggs would do a better job than we can do at home and from what I remember about their pricing it certainly was reasonable. I just thought that this method on this video was so easy and so inexpensive it might be worth a shot. I will probably try it just for my own curiosity. If it does not produce good and lasting results, then off to Boggs they will go.

I recently purchased all new (old stock - made in USA) Nicholson files and tossed out my old ones. I even bought some back-ups as the prices were still reasonable. The old ones were so far gone I did not even bother to try and restore them. I should have kept them (hindsight) to make knives out of but at the time I did not think of it.

Anyone interested in getting the last of the USA made Nicholson's left, should go on-line (Fleabay) and get some. The last of the Mohicans are dwindling fast and the current ones made in Mexico are NOT the same quality as the old USA made ones - that's for sure!

An old trick that many of you might know is to always rub chalk on the cutting surface of the file before using. They clean much easier with a file card and make a smoother cut and seem to stay sharper much longer.