IMO you get what you pay for in paint from my experience.
If the paint is coming off on the shells, some guys will use a plastic primer paint like that used on auto plastic bumpers for a good bond and then paint them with the finish colors. This way it will be the most durable paint adhesion. Just remember NOTHING is meant to polyethylene from achemical engineering stand point, which basically most bodies nad heads are made out of. Either way, if you use a primer or not, wash them to remove all dirt etc, and let thoroughly dry. Then use a soft sanding sponge of 80 grit to roughen up the surface of the old paint/plastic for best bond, then start your painting process.
I hope this helps out anybody doing repaints.