I don't know if there is a specific "look" that you're going for, but the one time I tried heat-bending metal, and I sprayed plain water on the hot-spot to cool it faster, the resulting mist covered the whole rest of the project with rust. Seems like if your goal is to rust the metal, plain old H2O will do the trick. Maybe a little too well!

To be fair, it's possible there was a bit of bleach residue in the bottle, which may have made a difference.