Proper prep is the key in painting factory non painted pieces. The following would need to be done. A good scrub with a grease and wax remover, there is a gel type with microscrubbers for body shop use (SEM makes one), then you need to sand all the texture off with a DA sander, then prime the part using a adhesion promoter made for flexible surfaces. Then it will need a flexible plastic primer or even better yet an epoxy sealer which arguably provides the best adhesion on flexible plastic pieces. Lastly, it will need approx 2 coats of base and 3 coats of clear followed by a wet sand and buff. I would suggest using paint from Dupont such as Spies Hecker, Standox or Glasurit, or Sikkens, which will ensure a very good match in my experience. I try to go to bodyshops and ask what materials and paints they use, that tells me the level of quality they try to achieve. I personally like a 3M paint preparation system and a paint system that use one of the above.