Shellac Solution 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 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).
Allbäck's Shellac Solution is a premixed solution made from pure shellac flakes and ethanol (99.9%) to the consistency of a thin syrup. It is very quick to dry (within 5 minutes). Apply as needed to adequately seal the surface.