No primer on rear bumper?

[edco]

Not sure where to put this so if this is in the wrong place my apologies..

My Huntington Green 06 X5 rear bumper (with PDC for when you get too close) got severely gauged by some a-hole who was too lazy to turn his/her steering wheel all the way to get out of the spot behind me.
Add to that my disregard for above referenced beeping from the PDC, and I put a nice deep gauge in it on both corners.

So a friend of my who works in a BMW dealership had one come in and the rear bumper (Alpine white) had the tiniest crease just below the PS tail light. So they replaced the bumper cover with one from the factory, and I got the one they took off.

Yesterday he was kind enough to do me a favor, put my X on a lift and change my scratched and gauged bumper for the like-new one.

Here's the question: When I spoke to a trusted Body Shop they advised that the bumpers DON'T get primed. They scruff them up a bit all around and then spray with the matching paint for my car.

Something about the bumpers being made of a plastic composite which would cause primer to flake underneath the paint even from a minor tap from a grocery store cart, not to mention the parallel parkers from hell.

I thought that bizarre - no primer on the bumper and thought I would ask here since my buddy's away for the weekend.

Thanks as always
ED