Well, I would paint a black undercoat, put on the metal and highlight as normal as if you didn\'t want rust. Then take some very diluted with water browns, and \"streak\" them down the armour or vehicle parts. I would try and get more of the brown \"ink\" in areas where water would naturally collect and stay or where rust would naturally occur (on undead armour then possibly rivets / bolt on armour, \"rims and lips\" on helms etc). If you use GW paints then flesh wash may be a good colour to start with as its a kind of \"rusty brown\".
If you want to do NMM with rust then I haven\'t a clue how to do that hehe