My problem: I'm customizing a TRU Transformers Masterpiece Rodimus Prime. I've sanded, primed, light sanded, and painted all the pieces. The paint on the yellow spoiler (the yellow triangular thing on the top rear end of the car) started cracking after I sprayed on the second coat. My solution: I'm going to sand off the cracked paint. Then spray on 1 or 2 new coats. Should I prime the pieces again before I paint them, or not bother with priming?