Gunner fired this volley in
:

Gunner, back to file carding aluminum from some files (GACK!!) and a
bit of rust on one or 3

You never heard of chalking a file to prevent pinning?

If you have any use for a file at all, put a handle on it, put a hole in
the handle, and hang it from the hole -- with a plastic or cloth scabbard
to help keep it clean.

Keep the rest rolled up in oiled paper -- just like Nicholson ships 'em.

Acid 'sharpening' takes a real knack, and isn't a good sharpening,
anyway. In practice, you run the file flat and backwards over a piece of
oiled fabric stretched tight around a sharp edge of a table or block.

That oils just the tips of the teeth as a sort of "resist" for the acid.
Hydrochloric acid dissolves iron (muratic).

You etch JUST the right length of time with JUST the right amount of
agitation to undercut those protected edges.

It never worked for me, and to hear old guys tell it, it never worked all
that well, anyway.

Lloyd