FYI, I treated 30 #2 brass screws with Birchwood-Casey Brass Black this morning (putting them in a small glass, adding solution, and agitating gently for 5 minutes, then rinsing and drying between two sheets of blue shop towels).
These are the best 12 from that batch.
A second treatment might get me a better yield, but I’m probably going to have to go back and spot these with a q-tip because the thin layer of black doesn’t stand up to the abrasiveness of a screwdriver.