Real rust is a great medium to work with.... I saw some AFV modellers do amazing things with it. I even tried it myself and I had amazing results (to my stsndards, at least

).
To use rust, collect it as Lindsay has told, the longer the wool steel stays in water the darker the rust will be.
Overcoat the part of whichever color you need ( i.e. black-brown if you want an overall rust finish or, using the salt tecnique, black-brown plus any base coat if you want a partially rusty surface), then spry the part with some sticky medium (hair srpy works fine with me) ans sprinkle your base rust (I use a pretty dark one, the shadow of burnt sienna, let's say). When it is cured you can go over it in a random pattern with lighter or darker shadow of rust. I usually range from orange to black. For this purpose I use also pastel powder, works great.
Give it a try, not that difficult as is seems.
I'll post some pictures soon...
Ciao
Edo