Painting ork armor can be so easy, even an Ork could do it!
1) Build model, duh.
2) Prime with Krylon Matte Aluminum. Matte, never Satin or Gloss.
3) Use a blister sponge (not kitchen sponge) to apply rust. Go crazy! I used Vallejo Rust Brown.
4) Wash, wash, wash the model with black wash. I used a whole pot of GW Nuln Oil.
5) Pick out details like glowy bits, rivets, and such.
6) Wash it again! I have used brown and black wash this time only on the newly painted parts, or spots that I missed.
7) Make an interesting base. Tell a story. I pulled out some cast away bits, and worked in a weedy grot.
8) Job's done!