This was "art-weekend"--eight women in a cabin on the lake, with nothing to do but eat good food, drink wine, and make art. One of the projects was "rusting". Most of the others were doing it on fabric, but since I'm much more of a paper person, I gave it a shot on watercolor paper. It worked really fast, with steel wool, and vinegar and water mixed together in a spray bottle. It wasn't really this red, but that's how my scanner scanned it. Then I used pen and ink, making the images that were suggested in the rust. The top one I like, the bottom one I don't (I did for awhile, but then made too many marks).
It's a pretty cool process, one I can see applying to a picture AFTER doing some kind of initial drawing, so that one can make the rusting happen in the places that you desire it.