This is a quick recipe I use when I want a simple but effective looking oily, rusty metal.

1) Mix up Orange Foundation Paint + Boltgun + Black (3:2:1), get a large brush and liberally plaster the area.

2) Drybrush Black + Boltgun over the previous layer

3) Do a very light drybrush of pure Boltgun over the raised edges and voila

For a final touch wash the crevices with Devlan Mud, followed by Badab Black.

Here’s the same technique applied to some in progress orks.