25 Sep 2012
Before we start painting it is important to know how rust looks. Just look up some google pictures and you'll find out rust has tones from very dark brown to very light orange. If you look a bit better you'll discover the fresher the rust is the lighter it is. this means you'll mostly find lighter orange in gaps where water is able to make pools and you'll find darker tones on sharp edges because things that pass the object will remove the fresh rust. (I hope you know what I mean.) I think this picture shows pretty much what I mean:
Now let's just start painting:
Step 1: paint the whole surface in scorched brown

Step 2: Make sure the scorched brown is dried. Thin down some scab red en wash the whole surface. I'm not sure about the ratio water/paint but I think it's about 3:1

Step 3: Make sure the scab red is dried. Thin down some blood red and apply it to the surface. don't cover the sharp edges and make sure to fill the gaps. Again water/paint ratio will be around 3:1

Step 4: After the blood red dried thin down some blazing orange, use less water then before, I think 1:2 would do it this time. Apply it to the full surface. Before the paint dried gently move your finger over the surface to remove some paint. The orange will stay in the gaps and will be gone on the edges after this.

Step 5: Make sure everything is dried again and thin down some fiery orange in a ratio of 1:3 of something like that. (I really don't know what ratio I use. I just thin it down till it looks ok to me.) Apply the fiery orange to some particular places that will naturally show the lightest rust colours. I added it to the places where the bolts and gaps show up. Again use your fingers to remove the paint that is to much before it dried.

Step 6: take some chainmail and apply a very light drybrush. (I think this already is too much.)

Done? great. You now can add some pigments to make it even more realistic but I hate pigments as you can't touch your model anymore after adding it. This looks ok for me.
Finally I'll like to show you this link:
http://www.scratchmod.com/Home.php
No warhammer but still some very nice wheatered vehicles.
I learned a lot from the tutorials on the site but they seem to be down at the moment. I hope He'll fix that once.
Good luck!