First, you will need to rough it up a bit with sandpaper to get through to the metal. A lot of people, I think, use salt water which takes forever on its own. You need an oxidizer to speed up the task, so, in order to achieve a uniform, thick rust quickly (seriously, like, 5 minutes or less), mix up a batch of the following:
half part hydrogen peroxide (drugstore grade is fine), half part vinegar, and a pinch of salt (I mixed it in a spray bottle and sprayed it on!)The method is, uhm, rather fast, because of the chemical reaction that is created. It was suggested that you spray a coat of lacquer when it's dry, so I guess you could, but I didn't. I want it to look like it's been weathered and mistreated!