Do like you would on a real car body. I would first wet sand it with a very high grit sandpaper (Very very high grit and fine, to dull the plastic with minute scratches. Put the body under slow running water and sand it with light pressure) and then wash it thoroughly. Then put a primer coat, several color coats as necessary and then a few clear coats.
I used Dupli-Color spray paints at Advanced Auto for my Tundra. It turned out really good too and a lot cheaper than Tamiya paints. Do not overspray and let each coat dry completely, wet sand every coat very lightly before application of the next layer to give a good adhesion surface and to remove impurities.
You will not cover all the areas in one attempt, so it is better to apply several very light coats instead of one heavy coat. Start spraying outside the body first and then across the body in a constant sweeping motion. Otherwise you'll end up with too much overspray and the dreaded 'orange peel' look.
Be patient and Have fun.
These are the results: It took me around 5 days to complete the job.
This is with the primer. I chose a gray colored primer so I can distinguish between primed and unprimed areas easily.
Color coat without clear coat.
After 3 layers of color and 2 layers of clear, gives a nice rich metallic blue sheen. Almost show-room finish.