You are right about it being rock hard when dry. That’s why lacquer can be buffed to a high gloss. The harder a substance is, the brighter it will be when polished. For example, the glass of a mirror, or the luster of a diamond. However, lacquer is not as hard as some of the urethanes that are mixed with a catalyst.

Lacquer has an advantage over many coatings because almost all the solvents work out of it in about 6 months, whereas spar varnish, synthetic enamel, shellac, etc., may take many years. Actually, synthetic enamel never completely dries on an automobile unless it is baked at a high enough temperature to melt the tires off it and burn the upholstery out of it. So much for the typical Earl Scheib bake job. Manufacturers such as Ford baked enamel at a high temperature on just the sheet metal though, which gave a fairly high gloss. In contrast, GM used lacquer and dried it with just enough heat to completely get rid of the solvents. They both held up about the same, so lacquer is not necessarily better under some circumstances. What all this means is that we can work with lacquer at home and still get very good results without baking. However, when lacquer air dries, it continuously shrinks, and imperfections can begin to show. This is why some lacquer finishes must be buffed after about 6 months.

On the other hand, shellac cannot even be cured in the hot sun, and takes many years to completely dry. If it was originally applied rather thick, it will crack instead of shrink. Even though the cracks can be very small, they prevent you from seeing through the finish, and this is the reason why it begins to look almost black. However, old shellac can be rejuvenated to eliminate these cracks and bring back the finish. By the same token, lacquer can crack over time if it was applied on a hot day or if the humidity is too high, and it can also be rejuvenated to bring back the finish.