Used most often to finish luthier-made classical and flamenco guitars, French polish is surrounded by more myths and opinions than virtually any other aspect of guitar making. French polish has been a common furniture finish since before the Victorian era and was probably first used on instruments as a faster-drying alternative to the very slow-drying varnishes used on violins. French polish shines with a delicious, organic, old-world elegance, and bonds perfectly to wood. Because it is extremely thin, it allows for the best possible tone, and it can be easily repaired and reconditioned at any time. Let's take a look at what French polish is, as well as its application and care.
HAND-APPLIED SHELLAC
French polish is simply a shellac finish that is applied by hand—rubbed on with a small cloth pad. Shellac itself is a resin that comes from a secretion of the South Asian lac insect, which feeds on the bark of trees. 
Color variations in the shellac come from the variety of trees the insects feed on. The secretions are crystallized into flakes, like thin peanut brittle, which are dissolved in alcohol to make liquid shellac. It is mixed in small portions because liquid shellac has a relatively short shelf life (about six months). Most luthiers come up with their own mixing system and thin it according to need and the way that a particular batch of shellac behaves.
The main reason luthiers use the French-polish technique is to cover and protect the instrument using the least possible amount of finish. To meet this goal, the shellac is rubbed on by hand one ultrathin layer at a time, using a pad made of a piece of linen wrapped around a wad of cotton or wool. Although it generally requires the application of several hundred layers, French polish is still thinner than most other finishing options.
FILLING THE PORES
The process begins with the application of a thin wash coat of shellac to the whole instrument (using a pad). Then the chosen filler is applied. Porous woods, such as rosewood, need to have their pores filled, which is commonly done with commercial paste fillers, epoxy, or the old-world solution of pumice. Pumice may seem an unlikely filler since it is a powdered stone that looks totally white, but when suspended in shellac it becomes translucent and absorbs the color of the wood as it's rubbed into the pores. Pumice is like lightweight powdered glass and has a hardening effect on the surface of the wood. This effect is partly responsible for the clarity of sound a French polish gives to an instrument—pumice isn't just pore filler, but becomes part of the surface of the wood, which affects the instrument's resonance. Pumice or other fillers are never used on soundboard woods like spruce and cedar, and other less-porous body woods such as maple or cypress give a beautiful finished surface without the use of fillers.
HUNDREDS, PERHAPS THOUSANDS, OF LAYERS
Once the pores are filled, liquid shellac is applied with the pad in small circular or figure-eight motions; in this way, the luthier continuously adds, spreads, and smooths a tiny amount of shellac resin dissolved in the alcohol. Sometimes, but not always, mineral, linseed, or walnut oil is used to lubricate the pad and keep it from sticking and marking the finish. As the alcohol evaporates, the resins are left behind. 
This process continues until hundreds, or even thousands, of microscopically thin coats are in place and the desired finish achieved. The work is done in sessions of about a half hour, maybe three sessions in a day, then repeated in a day or two, perhaps taking three days total. It is important to learn when to work and when to rest, to find the optimum balance between building coats and drying. It's hypnotic work and, with the smell of the alcohol, quite pleasant. Once cured, it is possible to sand and buff French polish as you would lacquer, but it must be done very carefully, because shellac is much softer than lacquer.
There are many debates among luthiers about which oil, alcohol, shellac, filler, or kind of pad to use—it's almost scary how strong opinions can be. There is also a wide range of thought on how long it takes to do the job: a day, five days, a month, a year. In my shop, the finish is applied in about three days and cured for two to four weeks. (The longer the cure the better, but we're anxious to play the instrument, not just look at it.) In a pinch, I've applied a complete French polish finish in one day, let it dry another day, strung the guitar up, and hopped on a plane without a problem. I don't recommend it, but it can be done.
CARING FOR FRENCH-POLISHED GUITARS
French polish does have some liabilities. It scratches easily, especially on soft woods, such as spruce or cedar. It is also susceptible to heat, which can soften it. A guitar left in a car above 95 degrees Fahrenheit will probably receive an imprint from the case or, more disturbing, may stick to the case lining. Moisture can cloud the finish, and the body chemistry of some players can cause problems, crazing the contact surface over time. For someone who has experience using French polish, repairing the finish is an easy job, but using a method appropriate for other finishes can be a disaster. A poorly informed repair tech might use some finish other than shellac to do touch-up, might sand too aggressively, or might not have the skill with the pad and the shellac to blend, fill, and smooth the finish. At the same time, someone with the right skills and experience can often renew a French polish in minutes.
To take care of French polish, lightly rub with a soft cloth to clean it, and avoid the common enemies of heat, moisture, and fingernails. Also remember that small flaws and damage can be attended to any time, so don't be too worried when something does happen—it can be dealt with later.
FINISH OF THE FUTURE?
As guitar makers continue to look beyond their own niche for ideas, French polish is increasingly showing up in place of other production finishes. Steel-string luthiers such as Michael Bashkin are offering French polish as a finishing choice, and even the Santa Cruz Guitar Co. is offering a French-polished soundboard option on its Tony Rice model. It is fairly common to use shellac on the soundboard only, but using it on the entire guitar is the norm for classical guitar makers.
Me, I just love the stuff. I love how it looks, how it feels, how it smells, how it sounds. My company has used it on hundreds, maybe thousands, of guitars, and I don't even own a spray gun anymore. I expect that in the future this small renaissance will grow, with more luthiers and players becoming comfortable with the small miracle of French polish.
Kenny Hill
This article appeared in ACOUSTIC GUITAR magazine January 2009 issue 193
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