Well, I just painted my first stock. It's plastic. First, I filled in all the checkering with Bondo, and sanded the entire thing down with 80-grit. Then I smoothed out the sanding marks with 180 grit, and gave it a couple coats of plastic primer. Then I hit it with a coat of Canadian Tire brand spray-on truck bed liner. Once that dried, I primed it again, and applied a finishing coat of krylon matte 'desert sand' camo paint.
It came out quite nice, the texture's great. Problem is that it does scratch fairly easily - but that's also easily corrected with a shot of desert sand paint afterwords.