The way i do it with good results is to paint the pieces outside by hanging them on a clothes line with a foot of 14 gauge wire then i spray paint them with a thin coat about every half hour.
First make sure you are using the proper paint, made for plastic rather than metal models. Then try using a very fine brush so that you can do detail work without it looking sloppy. There may be an airbrush that could be used to do the first coat, then do the detailing with your fine brush.
I've always brush painted, but I use small amounts of paint, spread thin. I use two or three thin coats on main parts. Then I go over details as needed. I usually do a wash afterwards of black water-based paint diluted in water. This adds a more realistic finish.