I think you're rushing your paint coats by applying the paint too heavy. Some colors can be real touchy about cracking and when I encounter this issue I thin down the pain a bit and use several thin coats that are heat set between each coat. Another thing to think about is to give the blanks a good wiping with a paper towel/alcohol after you've wrapped the bills and allowing them to dry well before painting. Then be careful to handle them by the bill if the blanks have one.
