A home-made "parkerized" (phospated) finish is relatively easy to do.
I've had good results from using Naval Jelly rust remover and water (50/50 solution) in a campstove heated stainless steel cooking pot. Add some iron filings to the mix. Just bring it to a slow simmering boil and immerse your part into the mix. Keep at a low boil for about 30 mutes, remove, rinse, and oil.
Remember that you are working with a hot phosphoric acid solution. USE EYE, FACE, ARMS, AND HAND PROTECTION when doing this!