Try sandblasting it to open up the surface of the metal. If that’s
too fuzzy, wire brush it down, but at least get the mill scale off so
that you’re working on clean metal. Navel Jelly will work too if you
don’t have a sand blaster. It’s a jellied hydrochloric acid available
in hardware stores. Once you’re working with clean metal, apply a
ferric nitrate and water solution on the heated surface, not so hot
that the solution bounces off, just so it steams when it’s applied.
And keep your face out of the way, put a fan on it to blow the fumes
away. Not poisonous, just corrosive and not good to breath. Keep
heating and applying, you’ll see it rusting, but some of that is the
actual ferric nitrate drying out of solution, so keep at it for at
least a half dozen applications. Then let it sit for a couple days.
Neutralize the reaction with ammonia when you’ve got it where you
want it. After that, I like to rub it down with a thick application
of Johnson’s paste wax, then buff it up. Nice leathery brown look
when you’re done.
Another solution, which I hesitate to recommend, is Plumb Brown
Barrel Finish, available from gun shops, (off hand I can’t remember
the manufacturer). Be REAL careful with that stuff as it’s got a
mercury compound in it and it’s quite toxic. And the various barrel
bluing compounds contain selenium, also deadly poisonous. The blue
color is actually an iron oxide, the first one formed. Ferrous oxide,
I believe. The brown is ferric oxide, which forms after the blue. (I
hope I don’t have those turned around though, I get too many
opportunities to expose my limited knowlege of chemistry).
David L. Huffman