You will need to do a fair amount of prep...the plastic type nature of these tends to keep the paint from adhering as well as with other surfaces. Vigours cleaning, TSP rinse, light sanding and good binding primer would be my minimum recommendation.
wow...it may not be worth it! I do really well with the paint on a wood surface ;)B
Haven't personally tried it, but I've heard Glidden's Gripper primer works wonders on all surfaces. Best of luck.