I vote vinegar. I manage a park with a historical village. My current building under construction is a jail. I built my bunks last week and intend to hang the outside edge by chain. No old chain though. Bought new, bright, shiney, zinc coated. Was going to throw it in the coal forge but it was so flipping hot (let alone the fumes). Plan B kicked in. 99% food grade vinager (use it at my park's pool for cleaning hard water deposits). Covered the chain at 3p in the vinager. At 6am, the vinager was black and the chain was grey. Rinsed it off and in 10 minutes it was rusting. Quick coating of flat black and my bunks were done. Where did I learn of vinager...here.