You should clean and prime the plastic before spraying it though, because dust and oils can ruin the paint finish.
You can buy sprays in 3 types. I would stick with acrylic, as it is the most abundant, and easiest to use. Also, be careful of spraying on a tree. I find that cutting the plastic off the tree exposes raw plastic that looks butt ugly on a finished kit. I use tooth picks/ BBQ pokers, and a foam base to spray most of the pieces.