Not to shit on everyone, but I would suggest you stay away from all of the above methods. Here's why:
Anything out of a spray can is usually junk, and you can't just spray any type of paint onto plastic and expect it to hold up.
As for fiberglass/gelcoat method, fiberglass does not bond to plastic...no matter how hard you try. Yes people do it all the time, yes you see it in magazines...but it just wont hold up. Try it yourself if you want. You can even sand the shit out of the plastic with 36grit then apply fiberglass. Let it harden for a year if you want, then give the plastic just a tiny flex or twist and boom...no more bonded fiberglass. (Yes this includes using SMC fiberglass)
Here's what I suggest: Whatever plastic your using, prep work is key. I would dry sand first with 180 grit, then again with 220 grit. This is to give the primer something to bite into. If you're real concerned with a flawless finish, wet sand with 320. Then clean with a prepsol cleaner, and prime with a quality high build primer. Wet sand this with 600 grit. Now for the topcoat it depends on just how durable you need it. For show cars that don't see abuse we use base/clear. But if the car sees normal use, I suggest a single stage enamel. If the car gets its ass kicked, go to an industrial coating like Duponts Imron. Here's links to a couple interiors we've done for examples of how it turns out:
http://www.evocustom.com/Images/99gr...egrandam35.jpg
http://www.evocustom.com/Images/89ci...arundone16.jpg
http://www.evocustom.com/Images/auto...oshow06-27.jpg