Sanding sealer works by penetrating wood pores and hardening. It is nothing more than very thick lacquer with heavy particulates. It is used to prepare plywood for a top coat and preserve it against moisture penetration. To properly use sanding sealer, all edges of the plywood must be coated. This is best done with an air compressor connected to a spray gun using a pressure pot, atomising the sealer as a fine mist. After the sealer dries, you can then sand it and apply top coats of lacquer.
Fill the pressure pot air gun with sanding sealer. Plug the gun into the air compressor.
Lay the plywood out across two sawhorses. Holding the gun at about a 30 degree angle to the plywood from about 8 inches away, pull the trigger. Walk beside the plywood spraying the sealer, applying a wet band on the plywood. When you get to the end of the plywood let go of the trigger.
Walk backward as you pull the trigger, holding the gun as you did before, spraying the wet band on the plywood. Overlap the first band by about 1 inch. Continue walking forward and backward moving the wet band across the plywood until the surface of the plywood is completely wet with sealer.
Hold the gun down beside the edge of the plywood. Pull the trigger and blow wet sealer onto the edge of the plywood as you walk beside it. When you get to the end, hold the gun down and spray the top edge. Walk around and spray the outside edge, and then the bottom edge. Wait one hour for the sealer to dry.
Turn the plywood upside down. Spray the bottom and sides again like you did the first time. Select the face side of your plywood. Using the sandpaper, sand the entire surface of the plywood until the face has a white powder all over it. You can then spray on another top coat of sealer, or spray on a top coat of lacquer, varnish or shellac.
If you are finishing hardwood plywood, start with the bottom side up. Then when you turn it over, the top side will be ready for final finishing without turning it over again. You can apply sealer with a brush, but your results won't be as good and it will take longer.
Always wear safety glasses and breathing protection.