Pretty cool! Like the damage... very creative.
I don't remember where, but YEARS ago I found a GREAT tutorial on air-brushing tanks and planes. After you finish the modeling and prime the model, the tutorial suggested airbrushing boltgun metal (they called it 'aluminum fuselage') over the damage... then you can put a mask-paint over the damage. This mask paint is some sort of latex paint that you put on the canvas (water color) or model to cover layers of paint. Then, you paint the model (no need to worry about painting over the latex) the base 'color'.
After that, you can paint cammo over, highlight, etc. When you finish, you remove the latex (you can simply rub your finger over it and it comes off) and voilá... scratched paint!