I know guys that have the set-up to do so like to spray paint. I always use a roller and put a few coats on (sometimes as many as 7), after I've primed the cab. I do some light sanding with very fine sand paper between coats.
As far as being prepared to apply sideart goes, I learned the hard way but not prepping the sides at all before and the art started to peel up on the sides. so someone suggested using alcohol (I know there is a type and a percentage but it escapes me now) and I cleaned the side of the cab with alcohol and didn't have any issues with the next set of sideart I applied.