When you have completed the assembly of your model, try to use metallic paint as your primer. It has a number of advantages:
Once finished (let it dry for at least a week after you have applyed the metallic paint, and another week after your final paint coat), you can carefully scrape away the top layers of paint with a blunt object (such as the end a pair of nippers, etc.) to give a realistic chipped paint effect. Just dont get carried away, as the effect quickly gets out of hand.
Try it on an old model first - it takes a little practice.
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