krylon makes a plastic "bonding" type of spray paint that may be good...but I havent used it as of yet.
Just make sure you pick a flat, non texture, non latex.
Flat enamels stick good, and allow for sanding and repainting easily.
If you do plastic models then by all means use your air brush.
The prepping is where its at though, nothing will stick, or it will come out uneven if its not prepped perfectly.
Most anyone can make it look nice, but after its been touched and used for a little while...thats where the prep / paint difference comes in. If the plastic has a texture, then wet sand it all before painting...otherwise I would just toss the case.