Shellac Flakes are integral for the window restoration process. It is 100% necessary to coat the rabbits (grooves) of the window sash so keep the oil from the glazing putty from soaking into the wooden window frame and drying out the putty over time. It creates a perfect barrier for this application.
 
If shellac is applied, your window using linseed oil putty and painted with linseed oil paint will last for 50 years. 
 
Mix the shellac flakes with a few drops of denatured alcohol. The consistency should be that of syrup and if thick enough, there should be only one coat needed. Drying time will be approximately one hour. 
 
Shellac can also be used cover knots in wood and avoid unsightly leeching of sap through the paint.