Lots of ways to skin that cat, depending on exactly what you want to end up with.
Here's one I used.
Started with cleaning parts with alcohol and applying BC Perma-Blue, then sprayed with straight bleach and let set for 15 minutes:
Then rinsed, steel-wooled, and re-sprayed with bleach, let set for another 10 minutes or so.
Parts came out of that step nice and rusty:
Scotch-Brite (green pad) and steel-wooled again.
Then dried and sprayed down with oil, and hit the parts with the MAPP torch until the oil smoked, let air-cool.
Finished with 600-grit sanding with oil.
just sand and steel wool (#0000) back to the level gray that you want.
For each batch , the whole process from start to finish was less than 2 hours.
Some parts showed a little more etching than others - I suppose that's due to different steel, or my timing was off on some batches.
Bleach contact time will determine the amount of etching/pitting in the steel. Experiment a little.
good luck !
/MM