And ... especially if runs/sags/whatever will be visible, take the door
off the hinges, lay it flat on horses, and then paint it.

Another trick is to drive big nails a small distance into the top and
bottom faces of a dismounted door, about as far apart as possible. Then
incline the door as much as possible with whatever else is available to
you. You can then handle the door by the nails, and get the second side
with min delay.

HTH,
J