Shellac, a secretion of the lac bug of S.E. Asia, contains a fair amount of natural wax. When using shellac as a finish in it's own right this isn't a problem but when using it as a sealer for some finishes, especially water based, the wax interferes with the finish sticking to the surface.
Up until recently it was necessary to dewax the shellac yourself. This was accomplished by just letting the stuff sit undisturbed. After some period of time the wax would settle out and you could pour off the dewaxed shellac. Zinnser has now come out with a premixed dewaxed shellac. The first, that I know of, on the market. Good thing too. A lot of good shellac gets thrown out with the wax when you use the settling method.
You can buy premixed shellac or shellac flakes and mix your own.
If you buy Zinsser bullseye they are stamped with a date. If you can avoid it, don't buy anything older then one year.
Do it yourself shellac comes in many shapes and grades depending on how well it had been refined. The least refined stuff actually has to be strained because it can still contain some tree bark and bug parts from the collection process. Pretty much any of it can be used for finishing anything, though some grades are preferred over others for particular tasks. Example, a fairly low level of refinement for restoration of old violins to match the original.
Shellac mix is measured in pound cuts. i.e. 1 lb cut, 2 lb cut, 3 lb cut. This means 1, 2, or 3 pounds of shellac has been mixed and diluted in 1 gallon of alcohol. One or two pound cuts make a good first couple of coats or sealer coats. three pound makes good subsequent coats if using shellac as a finish.
The more shellac per gallon the quicker the build if you are using it as a finish. I like a 2 lb cut for a finish and 1 lb cut for a sealer.
Regular Zinnser comes in 3 lb cuts. This can be made into a 1.5 lb cut by mixing 50/50 with alcohol. The new sealer shellac sealer is a two pound cut but still makes a good sealer out of the can..
Shellac, especially when dewaxed, makes an ideal sealer since it sticks to about everything and about everything sticks to it. It also does an outstanding job of sealing off the oil from exotics and sap and knots found in pine from the eventual finish. Including paint. This attribute makes it a good sealer to seal like based stain and finish from each other so you don't "lift" the stain when you apply the finish.
Shellac and lacquer share some common traits that, if they meet the projects protection requirements, make them a better finish then a varnish.
Alcohol is both a thinner and a solvent for shellac (lacquer thinner for lacquer). That means that not only can you dilute it with alcohol but you can remove it with alcohol. It also means that each subsequent coat of shellac will meld with the previous coat so you, in effect, end up with one solid coat that will not show witness lines (the lines that can appear on a varnish finish around an area where you sand through one coat and expose the next. Varnishes do not have solvents. Once they are cured, other then with strong chemical strippers or a lot of sanding and scraping, you can't wipe it off and it doesn't meld with the previous coat thus the need to scuff sand it.
Finally, shellac not only gives an exceptionally beautiful finish when you rub it out, it is an easy finish to rub out and the one I usually recommend as the first type of finish to try rubbing out on.
Hope it helps,
Mike Glennon
Heirloom Woods