Don't forget to wetsand, and buff for a really nice finish.
If painting polyurethane, treat the plastic, or 'sweat' it. You can use an Acohol or denatured spirits bath, or expose it to heat in your oven or direct sunlight. The latter step is used for fiberglass, which you don't want to subject to any sort of bath. Be careful though, as if the item is overheated, it'll change its shape.
Next, mix bondo and lay a layer over any irregular surface or dents. Then do a rough sand with....50 grit sandpaper. Next, once all major dents, and irregularities are gone, lay a layer or primer.
Sand with 100 grit dry, and patches will appear where the primer was sanded down first exposing any rises or irregularities you might have missed. Make sure to use contrasting primer.
Now spray two layers or primer and begin wet sanding. This will leave you with almost glossy surface. Once done, you can apply your first layer or paint. This one is more of a mist and will help to next layers to set properly. So spray from a distance and don't worry about covering it completely with paint. You should still be able to see the primer.
Wet sand again, only slightly, and then spray two to three more layers of paint. Once done, wet sand once, and buff to give you a glossy finish.
