John Norrell will do this for you for only $350.
Or, you can spend twenty bucks on a pint of moly-resin, twelve bucks on a cheap air brush from Michael's or Hobby Lobby and you're good to go.
The catch is, you'll need to prep the metal by gently (which means, don't go full-power, you only need to put a matte finish on it, not fully sandblast it down to China) blasting it with approx. 100-grit abrasive (sand, aluminum oxide, etc.). Most machine shops, welders and whatever have a blasting cabinet and a lower takes about five minutes to prep.
Blast the metal, clean with soap and water, heat in the oven to about 100 degrees and spray on the moly resin. The heating to 100 is so the carrier for the resin will evaporate quickly and leave a dry finish. Do this in August where I live and you won't have to pre-heat! Anyhow, once you have a nice coat (better, a couple of thin coats) on the piece, toss it in the oven and bake at 300 F for an hour and you're done.
This sounds complicated at first reading, but I've done a half-dozen lowers and it's really not only easy but actually becomes fun to do and is very satisfying when you pull that finished piece out of the oven and build your rifle.
Oh, and the smell is really minimal, my wife hardly noticed it.
John Norrel is in Little Rock AR and has a website - can't remember it exactly, but do a Google search on Norrell Moly Resin and it'll pop right up. He has Colt grey, matte black, semigloss black, stainless and clear finishes.
Have fun, feel free to ask me any questions.