Throughout History, instrument builders have used many different surface coatings to protect and visually enhance their instruments. Renaissance lute makers simply used egg whites to seal the wood and provide a slight sheen. Violin builders of Stradivarius' era guarded their complex varnish "recipes" and passed them down from generation to generation. More recently, luthiers have employed lacquers to add the final touch to their craft.
Much research has gone into developing a thin finish that will not impede the vibrational modes of the guitar, but still provide protection. Consider the following: the average thickness of an acoustic guitar top is .108". A modern guitar finish is typically .005" to .010" thick. In other words, the finish comprises almost 10% of the thickness of the top. Most finishes have a specific gravity (density) about 3 times as heavy as spruce, the most common soundboard wood. Therefore, a finish that is only 10% of the thickness for the top accounts for close to 30% of the weight!!! Because wood has a much greater strength to weight ratio than lacquer or varnish, the finish adds almost no strength to the top, merely mass. This additional weight can lower the resonant frequency of the top and reduce the amplitude of vibration.
Historically, luthiers have used thin films of varnish, shellac, or more recently lacquer to achieve ultra-thin finishes that protect without inhibiting sound. Quick drying spirit varnishes, such as French polish, are made by dissolving shellac, sandarac or other brittle gums in turpentine or methylated spirit. French polish (as used by C.F.Martin until 1900) is a technique that involves wiping on repeated applications of shellac to achieve a coating. The polish "rubber" (applicator) is made up of cotton balls wrapped in linen and soaked in a methanol/shellac mixture. A small amount of mineral oil is applied to the bottom of the "rubber" to keep it from sticking to the previously applied finish. Through successive rubbing's, shellac is deposited on the surface. Properly applied, French polish leaves a microscopic, high quality finish still favored by some classical guitar builders.
Oil varnishes, favored by violin builders, dry not by evaporation like spirit varnishes, but by oxidation. The oil in the varnish absorbs oxygen from the air and solidifies the oil with the gum. It is this oxidation and the resultant crystallization that can enhance tone by promoting lateral transmission of vibration. Since vibrations want to travel in the direction of the grain of the top, any finish that facilitates lateral transmission (across the grain) can greatly improve the sonority of an instrument. This type of finish has lost favor with some builders because of it's extreme slow drying time.
In the 1920's, nitrocellulose lacquer became prominent with guitar builders such as Martin and Gibson because of it's relative ease of application and fast drying time. Raw cellulose occurs freely in nature in the form of vegetable tissue, but historically one source has been favored for nitrating. Flowers of the cotton plant are rich in cellulose and after picking, what's left adhering to the pod ("linter's") is processed in a mixture of sulphuric and nitric acids which produces nitro-cellulose.
In general, nitro lacquers have 4 main ingredients: the cellulose derivative; a resin to increase build and luster; a plasticiser to lessen brittleness; a volatile thinner to facilitate application. The advent of nitro lacquer was the last piece of the puzzle that allowed mass production of guitars and started the sociological phenomenon in music we have experienced over the last 70 years.
Recent research has taken lacquers into the realm of catylized and ultra-violet cured coatings. Considering the guitar finishes profound sonic impact, I'm sure we will see many more innovations in the years to come.