Two schools of thought on rust (sad I know but bear with me)
1) Severe corrosion, built up over several years of poor maintenance, were the surface of the metal has flaked up ...this is the pastels and superglue/varnish approach.
2) Rust staining caused by suraces in contact with corroded chains running water, etc, i.e mild surface corrosion, this can be done with powders, or railway rust paint, or indeed oil paints, in the past for staining and general grime i've used ronseal wood stain, (dark mahogany), watered down(its acrylic) and applied as a wash and then smeared gradually thicker gives a good effect, if you over do it a wet cloth before its dries can remove it easily. a sample below of the effect, I know its a lousy picture but its the best I had at 7 am on a saturday morning......
Si..