Varnish, Shellac, Glaire
Although varnish has been used for centuries, the present-day guitars are either french-polished or spray finished with lacquer.
Jose Romanillos, The Classical Guitar, from Making Musical Instruments, edited by Charles Ford, 1979
I am always looking for ways to improve the finish on the classical guitars I make and I spend a good bit of time on the Internet looking for articles on French polish and violin varnish. Recently, I decided to revisit the use of egg white to seal the soundboard of a guitar that I am currently working on. I have blogged about using egg white with pumice as a pore filler elsewhere in this blog, I found out that egg white and pumice isn't the best pore filler, egg white is not water proof, contrary to some statements found on other websites.
Making an egg white wash is easy to do, you separate the white from the yolk and then whip the egg white until it forms soft to medium peaks. If you have done any baking, you will know what I am talking about. Then you place the egg white in a strainer, set the strainer over a bowl and let the egg white drain into the bowl. The straining is done at room temperature and it can take several hours, or over night, for all the glaire to strain out. Tradition states that the longer glaire sits at room temperature, the better it will be, but just be aware that the sulphide in the egg white will start to smell like, well, rotten eggs.
Glaire has been used for centuries to make water soluble paints and ink, just Google it and you will find plenty of information about it and its uses. If you research tempera paint, you will find recipes where all you do is mix the yolk with water and pigment, maybe a little binder such as flour, to make a waterproof paint.
Glaire is often used in wood finishing to help raise the grain of the wood. The guitar maker Jose Romanillos mentions in his book, Making the Spanish Guitar, that he once used glaire for that purpose, but discovered that it didn't raise the grain as much as water does. He does state that the egg white will give a spruce guitar top a nice patina. I have done that on several guitar tops and I agree, it does give the wood a bit more depth and a slightly aged appearance.
Here is the redwood/East Indian rosewood with a sanded top, without glaire...
and the top with several coats of glaire.
I will lightly sand the egg white sealer, then apply one or two coats and sand again before I start the French polish process.
Is glaire necessary? I don't think so, it is something that I like to use time and again. I have read that some makers have used it as a varnish, that seems counterproductive because egg white isn't waterproof. Maybe a person could make a varnish out of tempera...
As for the varnish that is in the title of this post, one thing I have come realize in all my research is that violin makers really know what they are doing when it comes to making and applying oil varnish. I guess comes from having four hundred and fifty years knowledge and tradition.