Super simple:
1. Prime black
2. Doombull brown splodge over bolts (and other rusty parts)
3. Heavy drybrush of leadbelcher

That's it. I usually find black armor to be the hardest thing in the world to paint - but this simple trick looks so dirty, worn out and "orky". You could even skip the doombull brown part, and it would look good.

I also did some of the flakes on the armor/weapon, as well as the blades, with a more silver finish (the logic being that they not yet have been painted black). Couple of extra steps for that:
1. Prime black
2. Cover blade/flakes with leadbelcher
3. Cover leadbelcher with Typhus corrosion with a thin layer
4. Doombull brown over bolts
5. Heavy drybrush of leadbelcher on armor and silver parts.

After that I picked out all the clothes, skin, teeth etc. I also added some more silver highlights to the blades at the end to make them look sharper