If it this type of effect you're after
Image from Scalemodelguide
Then you might be out of luck, I've added the tutorial but you would need to put the chipping layer underneath first before applying the top coat.
Chipping Effect
HOWEVER
You could try using a stippling sponge (Check fancy dress make up kits, Pirates especially, these sponges are used to do beards...Snazzaroo has something that might give you an idea) and carefully do it on the edges using the tiniest amount of paint (In case you make a mistake) but for the best chipping effects, I think you've gone way past that point unfortunately