I am not a painter but give a try. I hate drips, runs and sags in the finish. I have sprayed the doors vertically but sags are a problem for me, when that happens, I have to start over by sanding it smooth and respray. Now I paint the doors flat by setting up a temp "rotisserie" so you can rotate the door. The latest doors were solid and very heavy, this really helped. The setup is easy with some long pins or screws and a couple of sawhorses.