I am sorry but this is just plan wrong. I am a spray painter by trade and the material that we paint on does not make any difference to the quality of shine it is the quality of paint that counts. A lot of the high end cars like Audis and BMWs actually have plastic front guards, sometimes bonnets next to other metal panels and overall there is no difference to the shine of the paint from one panel to another. The substrate that paint is applied to has no bearing to the outcome of the "paint quality".