this method takes a long time, but works incredibly well... in small quantities
Find an old mouse (comp mouse, not the annoying furry things) and take out the ball. Cut the rubber "sleve" and you are left with a steel "ball bearing." Soak it in water for about 2-3 weeks, it will rust. then toss into a film can or other similar container. Get an old motor and somehow get it belt-driven. plug in moter. wait about an hour, you will get some EXTEMELY fine rust powder. rub where necisary. realistic because it's real rust. Have fun and don't use it if you have a cut in your finger.