Muratic Acid works great, just be careful of the fumes.
I use it full strength on gun parts.
We work with it quite a bit at work and in our shop.
Stripping old lead paints and varnishes off of milled, stainless or cast steel hardware from antique pieces being built in to new builds.
NOT FOR BRASS OR COPPER.....unless your painting over it again.
Before a Acid dip.......
I like to de-grease/ de-oil a part first, then dip into the acid. After it's stripped the old finish....I rinse it in water and blow dry ASAP. Final clean with Alcohol, M.E.K. or Lacquer Thinner then blue or park.
Had a Stainless Steel tank made at the local farm repair for $45.00.
Big enough for a barreled receiver. Smaller parts I use a smaller SS tank.
I like the Radocy Zinc Park. Use a 2 cycle oil with some green tint added for the after park oil bath.

Charlie-painter777