Shellac is a timeless complete to get premodern woodwork, and it therefore remains a major finishing choice for individuals engaged in authentic restorations and re-creations of historic wood furnishings, implements and fittings. The problem with shellac, as well as the reason it has fallen out of style as a modern wood finish, is when used alone it’s easily damaged by alcohol and water. To generate an enduring shellac finish, a sterile sealant has to be put on top.
Lay out a drop cloth or several layers of old papers, and set the wood object in addition to that.
Sand the surface of the shellac finish gently with fine-grit sandpaper on a hand sanding block. Sand the finish as much as is required to scuff it up a little with micro-scratches, making a better surface for your waterproofing sealant to bond with. Don’t try to remove the finish or some substantial quantity of wood.
Wipe the wooden object down with a tack cloth, to remove dust and pieces of sawdust and complete.
Apply a coating of translucent foam into the wooden object, with long strokes with a paintbrush. Wait several hours (or as directed on the instructions) before applying a second coat.