I’ve done a couple of experiments/tutorials on how to do rust, and left the easiest to the last: you can use rust you can buy as pigments.
Rust -iron oxide- comes in different colors; in fact it is used very widely as cheap pigments in a lot of applications from food coloring to arts and makeup.
You can buy these pigments (essentially powdered iron oxide) online. They can be mixed with each other, or with other pigments; they can be added to paints, and, if you’re brave enough, they can be airbrushed mixed with some sort of carrier or varnish/paint. If you mix them with paint or varnish, they will obviously stick to the surface; if you only mix them with a carrier (like white spirit, alcohol, even water) nothing will hold them onto the surface, so they will be quite easy to remove.
The method is simple: once you primed the surface with a dark primer (preference), you dab your brush into clear, matte varnish, dab it onto a paper towel to remove the excess, dab the brush into the iron oxide powder, dab the whole thing into a paper towel again, and now you have a brush loaded with a varnish/rust mixture you can use to gently deposit onto the model. You need to do it in several layers, and slowly build up the effect you are going for. You should start with the darker rust colors, and only use the bright reds/yellows sparingly; just check photos of rusted equipment for reference. (Come to think of it, you can go the other way around as well; just keep in mind that the bright rust colors tend to be somewhat sparse, limited to thin metals, edges and protrusions.) The surface you get this way will be suitable for depicting a very, very rusty metal surface: the metal plates of a derelict, burned-out vehicle, for example. (If you are going for mild, light rust, painting is your best bet.) Obviously there is always going to be a combination of rust effects that will bring you the best, more realistic results.
These pigments can be used suspended in white spirit or other carrier solutions as washes as well; they form a thin film on matte surfaces behaving more like filters, and they run into crevices on glossy surfaces behaving like traditional washes.
The colors can be further modulated using oil washes and paints; a dark wash will obviously darken the overall hues, and tie the different rust colors together.
The T-62 is a good example of the results.