+1 to what Sean (jfoodmaster) said. In my experience with hand painting vehicles, keep the paint thin (use at least a 1:1 ratio of paint to water), and be sure to get all the little nooks and crevices on the vehicle that tend to be less obvious. Give the base coat enough time to dry, and then retouch the places you might've missed. In all honesty, it won't take a lot longer to paint them by hand, since most only use an air brush for basecoating. The detail work is usually always done by hand. In the end, you'd probably only save about 5 minutes per vehicle.
Good luck!