Boiling in horse pee would work better. Walnut is good for cloth, but it's not an acid. Acid is the ticket. I apply my chosen acid, but not LSD, and then rust it in a sweat box.
Aqua fortis works, and specific browning solutions work well. The key is surface preparation and cleanliness. Oils on the metal surface will cause a spotty result. Too high a polish will also cause the acid to bead up, causing a spotty finish. 320 grit is all you need. After browning, neutralize with baking soda or ammonia.
Tom