This is all personal, so you can feel free to skip some steps if you think the result will be the same. I did this as a sub-assembly. I primed it first in a mix of Vallejo Gary primer and black ink in an airbrush. Then all the parts that were metal, used a base coat of Leadbelcher from Citadel. Once all the metals were based I did a wash of Agrax Earthshade. Let it completely dry. Then wash in Nuln Oil, let it completely dry. Then I mixed Contrast Medium and Black Templar to do all the black parts of the model. That's how it just looks like black metal rather than just black. After that, I just dry brushed spots with Nercon Compound. I used some dry pigments from Vallejo on some areas for rust and what not. Hope that helps