Shellac is available in two forms: as a liquid in cans you buy at paint stores and home centers, and as flakes you buy from woodworking suppliers. The advantage of dissolving flakes (in denatured alcohol) is to ensure the shellac you use is fresh. The fresher the dissolved shellac the quicker it hardens and the more water-resistant the dried finish.
Unfortunately, the sole remaining supplier of already-dissolved shellac no longer supplies a date of manufacture, so you have no idea how long the can has sat on a shelf or been stored in a box somewhere. When finishing important surfaces, it’s best to dissolve your own flakes.
When you do this, be sure to stir fairly often, at least once an hour. Otherwise the flakes will soften and stick together at the bottom of the container. They will form a lump that will be very difficult to break up.
Though this tip may sound silly and obvious, I know from my own experience that it’s not. The first time I dissolved shellac flakes, I simply combined them with the alcohol and left the jar alone. Next day I realized what I should have done.
To make the dissolving go faster, you can reduce the flakes to powder (for example, in a blender) or place the container in hot water. With both methods you still need to stir, however, until the shellac has dissolved.