All finishes including shellac take time to cure after application. Curing is complete after all the solvent has gassed out. The thumb rule for cure for most finishes is between 25-30 days. Even shellac, to show its true characteristics takes this period of time. Even though the finish may be dry to touch after a few hours, it does not necessarily mean that it has cured. A quick test to ascertain if it is ready for sanding before the cure time is to gently sand with 320 or higher grit. If dust is produced, it is safe to sand it.