Oak doors tend to have a more open grain than some other hardwood doors. For this reason, they need a slightly increased drying time. This applies to hollow-core, solid or raised-panel oak doors as well. Any lacquer or varnish can be used on oak doors, but if you use varnish or polyurethane, drying time will increase tenfold. Thus, fast-drying lacquer is recommended.
Lay the door out across two sawhorses in an open area with adequate ventilation. Lightly sand the surface by hand with the 220-grit sandpaper, with the grain. Turn over the door and sand the other side as well. Look for light scratches, sand them out, and blend the area by sanding over it again. Blow the dust off of the door with the air compressor. If you're applying stain, do it now.
Fill the pressure pot with lacquer and set the output pressure on the air compressor to 50. Test the spray gun by first spraying a piece of cardboard or scrap board with a short burst. Notice the pattern of the spray gun. You'll see either a vertical or horizontal "fan" to it. Adjust the fan to the pattern of your liking by turning the end of the spray gun.
Hold the spray gun 12 to 16 inches above the door at one corner and spray along the side of the door in a sweeping motion, walking along beside the door as you spray. Don't stop at any point. Make a straight band of wet lacquer along the door, including the side of the door. Walk backward, spraying as you go, just barely overlapping the first band of lacquer. Walk forward again with another band of wet lacquer. Repeat this forward and backward movement, spraying until the entire surface of the door is wet, including the edge.
Sweep across the end and the top of the door, making sure the entire door is sealed with wet lacquer. Let dry 30 minutes.
Flip the door over and spray the door again in the same fashion until it is completely wet; cover the ends as well. Let dry for 30 minutes. Using the sandpaper, sand the entire surface of the door again, including the edges. It should produce a white, powdery dust; this means your door is sealed and ready for the next coat. There is no need to remove the lacquer dust.
Spray the door again just like the first time including the edges, walking back and forth overlapping the lacquer bands until the door is wet and glassy-looking. Let dry for at least one hour. Flip the door over, hand-sand with the 220 paper and respray as before. Let dry overnight.
Stain on doors is optional. If stain is applied, wait the recommended drying time for the stain and proceed with the lacquer, which is the actual "finish."
Always wear a respirator when spraying lacquer.