You gentlemen are probably correct on all counts.
Although these parts are plastic, and flex, they of course aren't getting any flexure but hitting anything, but perhaps the reasons are due to expansion, and contraction of these parts as they heat, and cool?
And in regards to thickness pf application, again, probably correct, you'd maybe be spraying from now till christmas to achieve proper thickness of paint finish.
Wish there was a more simple solution, but it appears there is no easy fix to such parts, in comparison to professional paint shop results.
Thanks for your input folks, Mark