An instrument's finish is not just for beauty; its main purpose is protecting the fragile woods used in the instrument's construction from the ravages of the elements. Moisture and body oils can penetrate the pores of the wood, causing distortion in the wood's surface and damping the ability to transmit sound. Excessive dryness can cause cracks and splits in the wood. Even the sun's ultraviolet light can bleach the color from some woods while darkening and discoloring others. The finish on your instrument is designed to eliminate, or at least minimize, the effects the elements can have on your instrument.
The finish of your guitar can greatly determine the sound of the instrument. In comparison to steel-stringed guitars, classical guitars have weaker vibrational energy imparted to their tops (this is because of the use of nylon or gut strings). This is the reason that the best finishes are somewhat fragile themselves; the reason for this is that while the finish must protect an instrument from the elements, it still must allow the instrument to function as a finely tuned and delicate mechanism for sound production. Many instruments over the years have had their sound-producing capabilities destroyed by thick and tough finishes that did not allow the parts of the instrument to vibrate properly.
The three main finishing techniques that have emerged over the past few centuries (and are being used today) are: French Polishing, Polyurethane and Nitrocellulose Lacquer.
French Polishing
French polishing is a method of applying shellac to an instrument's surface. Despite the term "French polishing", no polish is used in this method of finish, and it is generally agreed that the term might have received its name from the extensive rubbing that is necessary to apply shellac smoothly and perfectly.
French polishing yields a very nice finish from an acoustic standpoint, but the finish is extremely delicate and can easily be damaged even by a fingernail. Due to the previously discussed weaker vibrational energy of classical guitars, the acoustic "transparency" of French polish is of particular benefit to the classical guitar. This offsets the fragility of the French polish finish, being the largest trade-off.
French polishing is considered by many to be the leading method for finishing Classical guitars, this is due to the labor-intensiveness of the job and the craftsmanship that is needed to execute the technique correctly.
Polyurethane
Polyurethane is a modern sprayed-on finish that is the most used finish nowadays. Acoustically, it might sound inferior to the above-mentioned french polishing technique or nitro-lacquer. Polyurethane has her positives however, she's fairly easy to apply and a fraction of the cost of other finishes. She can also be applied in a relatively short time and doesn't tend to crack or check over time.
The wrap on a Poly finish is that they are thick and inflexible, and don’t allow the instrument to vibrate and resonate as well as other finishes. But from a pure standpoint of manufacturing efficiency and cost, the more modern Poly finishes are the dominant method.
Nitrocellulose Lacquer
Nitrocellulose lacquer is a sprayed-on finish that is very durable and easy to touch-up, yielding great acoustic properties and excellent protection when applied properly. As nitrocellulose lacquer is a sprayed-on finish, specific techniques must be used: the finish must be applied in a sealed environment so that no dust or impurities are introduced, and spraying techniques must be precise and uniform to avoid imperfections. Nitrocellulose lacquer should be applied in coats, with hand-rubbing applied between coats for a uniform and thin final finish.
Early guitars used the Nitro style finish because that was the available material at the time for both classical instruments and electric guitars. Nitro finishes have many desirable qualities: They are thin, repairable, flexible, and don’t inhibit the vibrations of the wood. For a purely acoustic instrument these are highly desirable qualities, and a bad finish will kill the tone of an acoustic instrument. Nitro finishes are also labor intensive to apply, slow, are highly flammable, and toxic. They are a VOC (volatile organic compound) and subject to environmental limitations. Special permits are required to have a spray booth that uses lacquer finishes, which also raises the cost.
In all three discussed occasions, many variations are possible and this is something to keep in mind whilst purchasing (or building) an instrument. Feel free to contact us for more information on the use of these different lacquers and finishes.