Alright, there are a number of different ways to make armor look rusty.
1- if you want that rust look that you get from having left something underwater for a year, and then drug it out into combat, the best way to do this is to start off with the rust color. Use a reddish brown (P3 Bloodstone is perfect). You can shade it and highlight it a little as normal (a light brown ink wash might be all you need). But after you're finished with the color, take a bright silver and hard-line the edges. This is to show that some of the rust has chipped off, and there's a little bit of metal showing through.
2- if you want an older weapon look, I'd start out with tin bits, and highlight up to boltgun metal. After that, thin down your reddish brown so that it becomes a wash (again, I'd use P3 Bloodstone). You can apply this over the area several times, and even let it pool in places to get a more realistic rust pattern.
3- A little more tricky, to apply rust to larger areas or metal, make the same wash as in 2. This time, apply it in droplets to the area, and let them sit for a little. Clean them off before they dry completely, and you should get some uneven rings of rust.
4- A good way to pull a mini together with rust is to have streaks of it running down some of the paint. Just use the wash from type 2 and 3, and apply it in a line down from where you think the rust would be starting.