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. It is very quick to dry as the alcohol evaporates (within 5 minutes). Apply as needed to adequately seal the surface.
Shellac is an excellent general barrier. It is commonly used to seal knots in wood to prevent them from eventually bleeding through the finished paint. It is also used to seal the rebates of wooden windows before applying Allback 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).