Shellac is an excellent general barrier. It is commonly used to seal knots in wood to prevent their resin from eventually bleeding through the finished paint. It is also used to seal the rebates of wooden windows before applying Allbäck Linseed Putty to prevent the wood from absorbing the oil out of the glazing putty and drying it out prematurely (which is why old glazing always lasted for so long).
Mixing & Application
Pure shellac flakes can be dissolved in ethanol (pure denatured alcohol) or methanol (methyl hydrate) to form a viscous liquid the consistency of syrup for sealing surfaces. The shellac will dissolve in pure, strong alcohol (99.5%) in about one day. Weaker alcohol can take much longer - up to a week. It is very fast to dry, as the alcohol evaporates quickly. Apply as needed to adequately seal the surface.
For small projects using the 200 mL jar, add alcohol directly to the jar until the flakes are covered, and seal the lid.
For larger projects, where more shellac is needed, add shellac flakes from the 0.5 Kg Tub to a jar of your choice, fill with alcohol until the flakes are covered, and seal the lid.