It is a lot easier than it looks.
You spray the grey base. Then stipple ( get the paint to spatter on the tank. I use a popsicle stick and blow the paint off with my airbrush.) using black and white and if you want to another shade of grey. Then you freehand the cracks .
I have also done it with browns instead of grey for a sand look.
By the way Scott nice job on that one. It's a bit different than your usual style.
