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.

In step 2, do you mean drybrush a mix of black and boltgun metal or drybush one at a time?
The black is to darken the metal, so at the same time.
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