The metals were done by basing with boltgun metal, glazing with Thamar black to shade, and highlighted with chainmail and mithril silver. I created the rust by doing some light glazes with orange paint and ink, and then stippling orange paint more heavily in places.
The grittiness is a combination of the scratches I painted on, the rust I added, and my glazes not being nearly as smooth as they should be. :P