Way back when, Polly-S made a paint shade called RUST that I used with a drybrush technique. One little bottle lasted me a few years. Once it's dry, it doesn't rub off. However, the effect of painted on rust is quite different from dusted on chalk. The rust paint was especially good for simulating the way metal parts on armor vehicles have rust marks that drip down the side of the vehicle after a rain. That was not dry brushed, rather, I dabbed a spot where the rivet was, waited a minute or two, then streaked it down in the direction of gravity with a dry paper towel. The paint dries almost immediately, though, making touchup or re-do's of this effect almost impossible.