The principle differences between nitrocellulose lacquer and shellac are ease of application and their ability to block off problems in the wood. Both finishes are evaporative finishes, meaning that they dry entirely by solvent evaporation; there is no crosslinking as there is with varnish and catalyzed finishes.
As a result, both lacquer and shellac are more vulnerable to being damaged by coarse or sharp objects, heat, solvents, acids and alkalis. Shellac is more vulnerable than lacquer to being damaged by alcohol spills, of course, but keep in mind that beer, wine and mixed drinks are usually very watered down, so the vulnerability is much less than a straight alcohol spill.
Lacquer is much more user friendly than shellac because of the difference between thinners. Lacquer thinner is made up of a number of different solvents (usually about six, except in areas with strict VOC rules) that evaporate at different rates. This allows finishers to control the drying rate of the finish to avoid blushing, and runs and sags. With the right lacquer thinner, lacquer can even be applied successfully in cold temperatures. We have almost no control of the drying rate of shellac with just alcohol as the solvent.
But shellac has the advantage of being able to block off problems in the wood, such as silicone, which causes fish eye, various other oils, resin in oily woods, wax, etc. Shellac, therefore, can be useful as a sealer coat if you have one of these problems. For the most part, shellac is a great tool for refinishers who often encounter silicone problems, but of almost no advantage for finishing new wood.
The greater user friendliness of lacquer was the principle reason lacquer replaced shellac as a finish in the 1920s.
