Lay the door flat to avoid drips and runs
For convenience, it's tempting to leave a door hanging on its hinges when you paint. But for a smooth finish, you have to lay it flat and remove the hinges, knobs and other hardware. With the door laid flat on sawhorses, you can spread paint more quickly and not worry about drips and paint sags. And you can still paint both sides in a day if you rest the door on lag screws.
Drill one 3/16-in. hole in the bottom of the door and two at the top, then turn 4-in. by 1/4-in. lag screws 1-1/2 in. into the door. Spread the sawhorses apart just enough so that the door doesn't touch either side but rests entirely on the bolts. Paint the first side, then just rotate the door on the single bolt at the bottom of the door while holding the other two bolts.
Clean Off Grime Before You Prime
Washing your old finish is probably the most important step you can take to ensure good paint adhesion. Even the best paint won't stick well to oil and dirt, and there's lots of both on doors, especially near the knob, where dirty hands have pushed and pulled for years. Before filling holes or priming, scrub the entire door with heavy-duty household cleaner. Let the door dry completely, then fill any holes.