Rust is your friend

Hello,i've made in photoshop some test maps for little boilerbot.

For make this kind of maps usually i keep in a layer the original metal,and in another layer rust image,then with a kit of rusty,grunge,damage brushes and the erase tool i modify the original metal map.Is not difficult.(Obviusly you need a uvmap to paint)

Most used tools:eraser,dodge,burn,stamp tool and some patience.