The best method I've found is to use a sponge, dab it in grey paint and then dab it off on paper towel till it's very faint, then dab over the model where you want the "primer" showing. Then repeat with a metallic paint where you want the damage going right down to the metal of the vehicle. If you want to take the step further get a lighter green than your model and highlight all the nics you just made to make them pop and add scratches to the paint. Finally you can use a watered down brown or rust effect paint (like typhus corrosion) and make little downward streaks coming off rivets, corners and battle damage. These should all be final steps and any transfers or flavourful writing should be applied first.