Well... I can give you my rust formula...
1. Spray black
2. Dry brush blood red (or equivalent if you are not using GW paints)
3. Dry brush Blazing Orange, really light on this step...
4. Dry brush any yellow... Be very light... you just want to give it that tarnished looks, where the rust yellows in decay...
5. Put some chainmail or bolter on to have some freshly knicked metal around the edges...
6. Use some brown washes to wash out the bright metal and shadow the crevasses...
Here is a link to some models I did a ways back... That show the technique...
http://www.40konline.com/index.php?topic=215442.0Adam