I've done both, and you'll hear comments both ways. I think it depends on the color and what kind of paint. On my most recent build I painted the frame and floorpan black enamel with no primer and no problems. However if you're painting the floorpan the color of the body you may want to prime depending on the color, especially if you are using acrylics. The main thing is to wash the part first to remove any foreign matter to give the paint a good surface to adhere to.