I’m not familiar with a specific recipe for restoring old files, but
when mine get dull I generally clean and degrease them and then dip
them into dilute HCl for a while, watching the bubbles; when I think
the acid has done its work long enough I take them out. That’s fairly
ad-hoc but it does work. I’d also be interested in hearing about
specific formulations of acid combinations.
The principle is the same whatever the acid: to dissolve away enough
metal that the turned-over dull edges of the individual file teeth
become sharp again. You can only do this so often, though.