Ah, polypropyline, you just need a bit of flame and some alcohol!
Thank you, and I found the comments particularly interesting as well as the demonstration.
I have I have made several things out of high density polyethylene and poly propylene, and neither one takes paint well. Well I have tried sanding and primers of various kinds, but I never tried flame softening the surface.
I have done several experiments with heat forming, Flame polishing, and re-casting of these plastics. Specifically I was making sculpted risers for custom skateboards, and slider pucks for my skate gear.
I also made several U-shaped siphon tubes for my aquarium projects, by heat welding PP tubes.
I was going to paint this part (until I found out it was poly propylene) because the finish is non-uniform, like the guy that polished the dies didn’t really care about the finish on that side of the part being attractive.
Once my lamp is assembled, I will not be able to easily take it apart, because screws come from the inside of the backing plate and I am going to glue the lens on to prevent water intrusion. The surface, after trimming, was too irregular to allow the use of a normal gasket.
I am preparing to do the last disassembly, clearcoat the backing plate, and reassemble with RTV silicone, so this is my last chance if I want to paint the poly propylene.
I am still trying to fit some electronics into this package first. Maybe I will find something and get it together before Christmas, but we have lots of things to do and people coming over before then.