1) Clean the area well. I used brake cleaner with a wire hand brush. (Warning brake cleaner will damage most finishes!)
2)With a Q-tip(Not one you just picked the carrots out of your ears with.) apply some white paint.
3) After the white paint dries. Apply a dark color paint. (Try not to cover the stamped code completely)Let the darker paint set, but not dry completely.
4) With a toothpick gently remove the darker paint from the stamped code. ( You might find it easier if you cut the toothpick in half and use the fat end, which rides on top of the stamped code edges.)