Krylon Fusion spray paint works well on plastic, too. It's specifically designed to adhere to plastic. Krylon also makes an adhesion enhancing pre-spray. Here's my 10/22 with a Krylon Fusion flat camo tan painted Shooters Ridge synthetic thumbhole stock. I spent a lot of time on prep (light sanding, wipedown with acetown, washed in hot water), and the reward was a very durable paint job. I've since used it on handgun grips and other smaller plastic parts. It works.