A lot depends on what exactly you mean. For deep bubbling rust, Schnobs has luck starting with Mr Surfacer to get the texture.
Mig makes two pigments that are powders, a light and a heavy. they are just brushed on. They also make a couple washes, they are new to me but also brush on.
Rustall is a one stop solution in a bottle that works well. Paint it on in successive coats.
Others mix up some ornage, brown, black, etc and paint it.
I'm pretty new so all I can do is recite, not reccomend.
Bill