The prep work for all plastic is pretty much the same whether it's an interior, exterior, or underhood part. The biggest problem most people have is adhesion; the plastic either has to have a chemical etch to get a good bond, or a physical etch (sanding scratches).
Start by cleaning the piece in a succession of solvents; the first one should be water and detergent like Dawn. Next should be mineral spirits. I finish up with denatured alcohol. Use lots of paper towels or rags, and keep going until they're coming up clean.
I will usually give a final wipe with lacquer thinner, but be careful in that some plastics will soften with that. (Sorry, I don't know which ones do and which don't.) If you want to keep the existing texture of the piece, your final step is to spray on some adhesion promoter such as Bulldog, and follow its directions for recoating.
If you want to smooth a textured piece, you can wet sand it with something like 400 grit and finish up with some 600 grit.
If you already have a piece that is factory smooth, you can help adhesion by roughing it up with some 600 or 800 grit. I would still use an adhesion promotor spray next.
Following the a/p spray, follow with a plastic primer or primer surfacer. Primer only if you're trying to keep the existing texture, primer-surfacer if you're going for a smooth final appearance. P-surfacer can be built up fairly thickly and wet sanded with 600 - 2000 grit papers to get a glassy surface before applying color coats.
For final coats, there are any number of brands that will work; I'm assuming you're using spray cans as opposed to custom mixed automotive paints for spraying. The cans will usually say for plastics on them. This really just means they're more flexible, because you're actually painting the primed surface (the primer), not plastic. These color coats may or may not need to be clear-coated, depending on what kind of paint it is and what kind of final gloss level you want.
For example, if you're using a basecoat/clearcoat system, you will spray the base color of your choosing and then follow it up with a clear coat in the gloss level of your choice. Instructions are on the can for drying or "flash" time between coats.
In other cases you may choose a color that only comes in gloss, but you want a final finish of satin or matte, and will have to spray a clear coat over the color coat. In this case you want to make sure the products are compatible, and spray a scrap object ahead of the real project; there's nothing more discouraging than watching all your work bubble up and craze on what you thought would be the final coat!
You can get expert results with just spray cans if you take your time; there are two things to do. Multiple thin coats instead of a few thick ones. And adequate dry times between coats before re-coating, sanding, or applying the next layer of paint.
