To finish a knotty alder door while preserving the natural beauty of the wood, SFGate suggests sanding and varnishing. The sanding process requires two sawhorses, 320- and 420-grit sand paper, and a vacuum cleaner and naptha for cleaning. You also need brushes and marine spar varnish for varnishing.
Prepare the door and place it on the sawhorses
Remove all door accessories including locks, hinges and latches. Position the sawhorses appropriately, and place the door on the sawhorses.
Sand the door
Use 320-grit paper to sand the door. Sand in the direction of the grain to avoid unnecessary hideous marks.
Clear the dust and clean the door
Remove all the sanding dust using a vacuum cleaner. Rub the door with naptha to clean off any fine-particle dust.
Varnish the door
Using a 3- to 4-inch brush, paint the varnish onto the door. Leave the door overnight to dry.
Sand the door again
Sand the door again, this time using 400-grit sand paper, to even out the varnish.
Varnish and sand (repeating steps 4 and 5) two more times
Varnish the door again and let it dry before sanding again. Finishing a knotty alder door requires up to three coats of varnish, according to SFGate.