Why is the paint on bumpers harder to polish than metal painted parts

[HotWhipT/A]

I've noticed on just about ever single car I've worked on, the urethane bumpers require more pressure, working time, and or a more aggressive compound/polish to accomplish the same finish as say the surrounding fender, hood, trunk or quarter panel.

Why is that? My only guess is because of the flex additive they put in the paint, but why would that make such a difference?

 

[Flash Gordon]

I think it is a mixture of the flex agent, material being made of plastic and the funky angles you have to face when dealing with bumpers

Great question!