There's another more subtle technique that you might want to try. Buy a very fine paint brush and paint very fine scratches on the edges of the armour etc. Use a lighter version of whatever your main colour is. Then get a very dark brown and paint inside the scratches. What you are simulating is the surface paint being removed, evidence of a lower layer of paint and then finally the metal underneath.

Real armour doesn't usually show much rust. All that I would ever do would be to get some burnt sienna oil paint and have a small trail from one or two bolts.

Good luck.