Bob,
There's a whole heap of stuff on the subject. Mostly Hydrochloric acid was the stuff used.
Basically, it and phosphoric acid make a really useful rust remover on things like car panels.
Again, it is the stuff that is used to clean mortar , concrete and bricks as well so it is still available widely.
OK, moving on! I think that it is generally agreed that acid 'sharpening' is one way to get the best out of clogged up and rusty files but eventually, the teeth of files are bound to be worn away- end of story, I'm afraid.
Of course, it isn't the end for usefulness and the better( non case hardened) files make very good form tools for the lathe. Somewhere- somehow, I have photos of the files that Dennis Chaddock ground up to turn ball handles for his Quorn tool grinder.
Regards
Norman