paint problems

I'm no paint expert, but I believe when it comes to paint -- cleanliness is next to godliness. Especially on non-metal i would think contamination or non-compatible products would show. You might need to have the bumpers taken down to good known primer or even bare plastic and redone. :shrug:
 
You'll need flexible paint. Flexible gray shouldn't be a problem. Not all brands are equally good in this area either. I got mine at www.levineauto.com It wasn't 100% exact match but the quality was exactly what I was looking for. Don't be afraid to spend some money on flexible paint. You get what you pay for. I speak from experience. Good luck.
 
I talked to my local auto paint guys about this problem after I started having some trouble with my bumpers, not long after I painted the car. They said one way to go was to add a elasticizer to the paint and primer which allows the primer/paint to flex with the polyurethan bumpers/ground effects. I havn't tried it out yet so I"m not sure if it works, but its soposed to work with most paints.