Right, gotcha. Erm...magic, evil magic.
Seriously though sprayed Halfords primer grey. Its then drybrushed in a succession of lighter greys BUT as you will notice it has many flat parts with no relief. Standard drybrushing DOES NOT work on areas like this, it will just look streaky and crap. Rather than swiping the dry brush side to side it is kind of stabbed or stippled, its what creates that patina and it also creates light texture.
When dry (8 hoursish as I prefer enamels for drybrushing) a thin enamel black wash is applied. It automatically picks up the light texture. A brush moistened with spirit is then flapped about a bit to take some of it off.
The rust is model mates rust effect applied in strategic areas and blended out a bit, with water this time.
BTW the method is the same to do most types on concrete but on a base colour of Humbrol enamel 84 with grey added, using 84 plus white for the drybrushing, and adding green to the black wash. Also works with base of Humbrol acrylic 93, (93 plus acrylic white for drybrush) if you look at the pic the trunking is that...just to add some variation as there is much concrete and didn't want it all looking the same.
As I say....evil magic
