Auto body repair & detailing: Cracking & flaking of newly-painted bumper cover, ford windstar van, ford windstar

Question
A ford Windstar van had left quarter panel and rear bumper repairs - including R&R of the rear bumper cover with a new unit. A couple months later, a parking bump to the right end of the rear bumper has caused brittle fractures in the paint - with significant de-bonding and flaking of the damaged paint from the cover, with no adhesion `residue'. One of the body-shop owners peddled the lame line that "even new-car paint does that" (the original bumper's factory paint didn't crack or de-bond at all - with much heavier damage). It appears to be a combination problem of: Improper surface prep of the new cover (or improper selection of paint type), and failure to use/use enough flex-agent - what else could be going wrong?
And how do I convince the insurance carrier that the body-shop they certified did sub-standard work?

Answer
Neil-
While you may have a totally legit complaint here I am sorry to say that probably the warranty of the shop is not going to stand behind any failure of the paint due to an impact or damage. It is true in my opinion that the aftermarket finish is not going to be as resilient as the factory finish. The paint is just not designed to be as pliable as the plastic- regardless of the flex additive, it only ensure flexibility of the part for installation onto the vehicle. Sorry- Mark, Good Luck