This tutorial will have everything to do with completely rusting out something. As such it will be great for terrain, wrecks, and of course....Orks.
1. You want to prime your mini black, if you haven't then use a thick wash of black on it.
2 and 3. These come at the same time.
VERY IMPORTANT! Firstly put a thick wash of Beasty Brow (or another medium grade brown) over top of the area. You want it half way in between a wash and an actual coat. Only experimenting will give you the right results.
After you've laid that down get a red color (I used Gore Red) and water that down to the same consistency, use that to make blotches all over your area while the brown is still wet. make them different sizes and try to stick to the edges.
4. Give it a very thin black wash again, let dry and get a dark silver and add scratches to it, sticking to the edges. After this give it another thin black wash.
You will come up with something like so:
