I hope this helps. Im a body guy i work with this stuff every day. Bondo only sticks to bare metal. but there are some new products out there are made for plastic. Any glazing putty such as , Icing, Evercoat metalglaze, dolphin glaze will work but my suggestion is a product called polyfill. it come in squeze bottle and you mix hardner with it like a regular bondo. It lays out really smooth and will stick to any scuffed up paint. like a 180 grit or 320 grit. now if you trying to fill holes 3m just came out with a new product that is a rubber type mat with a sticky surface to one side. you can fill a hole as big as a golf ball and not worry about it. you use the mat as a backer and lay a plastic repair adhesive over it. after the plastic repair adhesive is cure you grind it off and skim it with polyfill. As for a plastic repair adhesive use 3m or Lord Fusor extreme bumper repair. Well I hope ive helped in some way and saved you the head ache of putting bondo over plastic. if you got anymore questoins just let me know.