Tony Immorlica replies: In my experience, shellac flakes last a very long time—years if kept in a dry environment in a sealed plastic bag. The only way I know to test shellac flakes is to make a solution—perhaps a 2 pound cut—and then brush it on a wood surface. The shellac should dry hard in about 10 to 30 minutes, depending on conditions in your shop. If it does not, or seems the least bit sticky, throw the flakes out.
Bob Couch replies: There’s only one way to tell for sure and that’s to mix a small 2lb. cut amount with denatured alcohol, let it sit covered for a few hours and then apply a coat to a test piece of wood.
Give it 24 hrs to cure and test it by sanding. If the paper gums up quickly, it’s probably not going to work well for you.