I went on a workshop recently...for rust dyeing on fabric. All you have to do is spray some fabric with a 50:50 mixture of distilled vinegar & water, place some steel wool or old, rusty metal (iron or steel) on the top and bind the fabric tightly with string or yarn. Place your bundle in a plastic bag and leave for a couple of days to 'rust'.
Unwrap, soak in a bucket of water/salt solution to fix the print and you're done. You can now use the rusty prints in your textile work!!!