You'll always get your best finish with your workpiece flat, but how to do both sides? Put two long screws in each end and rest the screws on sawhorses. Treat one side then use the screws to turn the doors and do the other side. You can even do it alone if you're careful. This is not a place for rickety old sawhorses! If you have a helper, you can let the finish tack over and then stand them up on the screws to dry while you do another door. Two sets of sawhorses will speed things up.

Don't rely on the screws to hold up under sanding pressure.