I think you may have put your finger on the problem. Spray lacquer is thinned (25% or more) to permit spraying. It could be that the spongy areas of the wood have a grain structure that prevents the thinned lacquer from "bridging across the cells". The shellac based sanding sealer goes on with a relatively thicker coat and since shellac is dissolved in alcohol the lacquer thinner does not disturb the seal coat. I will have to do some additional searching to test this guess. In any event, for me, a shellac seal is saving a lot of time and effort. For hang-on-the-wall plaques its durability should be more than adequate.