I take a toothbrush, dip it in prepared mustard, and then spritz the mustard over the item I wish to patinate.
The vinegar is the etchant, and the mustard seed paste is the indicator. Wash off with water.

To test, take a piece of mild steel, spritz it, leave over night, then decide. Lawnmower blade is a good choice for the test plate. It will freckle the steel nicely.

Out there doing it best I can.