Ever since musical instruments were crafted of wood they had to be protected. Whether it’s protection from the player (sweat can be quite aggressive; just take a look at the gold plating of your (friend’s) vintage Les Paul Custom) or from the elements, wood has to be safeguarded from hazardous,external influences. This is because the woods that are being used for instrument building are fragile. Some woods, like spruce, alder or basswood, simply can’t take much of a beating and are easily dented, bruised or maybe even worse. Other woods, like maple, tend to warp or maybe even crack under the influence of salts, acids, moisture or a combination of those influences. Some woods don’t need a thick coating, a thin coating will work (think of Musicman’s maple necks; they’re finished in gunstock oil to ensure a ‘raw’ feel but are protected nonetheless). Other woods don’t need a finish at all, such as rosewood, pau ferro or ebony.
In order to ensure the longevity of the instrument, the luthier covers the instrument with a special coating, the lacquer. The luthier (or the painter in charge) has a large array of possible materials. Let’s take a look at some of the major possibilities a luthier has when it comes to finishing a guitar.
Shellac, or enamel, was already used for hunderds of years on violins and acoustic guitars, this material is praised for its natural feel. A piece of wood finished in shellac (finished via the technique called ‘French Polish’), has a soft, honey-like gloss to it. The wood can ‘breath’ through the finish and the piece feels very ‘natural’. The downside to shellac is that it’s quite difficult to apply. You have to have some experience doing it. Also, shellac can show some wear over time, like some other finishes. The best part of shellac is that it’s an entirely natural product. It starts off as flakes that are diluted in alcohol. The flakes are a hardened resin of special lice. It’s being scraped off trees, collected, dried and then used. Interestingly enough, shellac is also being used as a protective layer on lemons and also as the insulator for the copper wires you find in your (PAF-style Seymour Duncan) pickup! Shellac forms, after it’s brushed on the piece, a protective film after the alcohol has evaporated.
Nitrocellulose was the next step up. It is in it’s raw form a highly flammable compound, used for explosives. This substance, known as guncotton, is being dissolved in acetone and then when it’s being sprayed or brushed, it hardens as a very thin film. Nitrocellulose lacquer was also used in the automotive industry. The acetone evaporates and the nitrocellulose makes a protective film on top of the wood. The nice thing about nitrocellulose lacquer is that it dries fast and polishes to a high gloss with relative ease. Also, when shooting in layers, the layers melt together to form one ‘thick’ layer (but understand that thick is still incredibly small: a guitar can have a final coat less than the thickness of a human hair).
A unique property of nitrocellulose is that it keeps on ‘fuming out’. By that I mean that the solvent continues to evaporate long after the coats are dry to the touch, maybe even dry enough for polishing. That means that the layer of nitrocellulose continues to get slimmer and slimmer as years go by. This is unique for nitrocellulose since other other paints don’t show this characteristic. That also means that nitrocellulose is quite fragile. It’s more prone to scratches than modern lacquers, but scratches are more easily buffed out than with modern lacquers.
Like shellac, nitrocellulose can show some wear if it was subjected to a lot of use (or should I say abuse?) Just take a look at vintage Les Pauls, these in nitrocellulose finished guitars show some bare spots in the finish. These spots are a direct result of friction. Also a valued trait of nitrocellulose is it’s ability to show craquelle. If the guitar is subject to rapid changes in temperature, the paint will expand and contract at a different rate than the wood, which will eventually result in cracks in the lacquer. Nitrocellulose can be tinted to any desired color, and it’s the dye that changes color over time under the influence of UV radiation (i.e. light) but the nitrocellulose itself, which starts out as simply crystal clear, discolors too. Where the dye will fade, nitrocellulose will yellow over time, adding to the lovely patina we all grew to love and appreciate.
In this day of being environmentally friendly, nitrocellulose can pose major issues. It’s not only very flammable when it’s still in its liquid form, but also slightly toxic. But that’s true for almost every kind of lacquer.
Acryllic lacquers behave very similar to nitrocellulose, except the solvent isn’t acetone but water, and the finish is a bit more brittle than nitrocellulose yet yielding the same aging characteristics that are so valued in nitrocellulose. Also, unlike nitrocellulose, the base material of acryllic is not flammable.
The modern lacquer of choice, polyurethane is an organic compound that is essentially a combination of large carbon based monomers. Polyurethane knows many applications, amongst others strong adhesives, synthetic fibers (Spandex) and varnish. Unlike nitrocellulose, polyurethane won’t shave off with wear and heavy use. It will retain it’s glossy finish, albeit with some (unavoidable) scratches and dings. Another major difference is that polyurethane is known to break off it’s substrate if subject to a highly energetic impact. If this happens, you will see white spots in the finish and they cannot be buffed out.
In its practical applications on guitars, polyurethane has to be shot in a thicker coat and can’t be shot in layers because the polyurethane doesn’t melt together. This is because hardening of polyurethane relies on a chemical reaction with the substrate, air or with chemicals in the lacquer, whereas other lacquers (such as shellac, nitrocellulose and acryllics) rely on an evaporation of the solvent.
A major point of criticism regarding polyurethane lacquer on guitars has not just been about environmental concerns but more about the durability of the lacquer. Polyurethane is so hard, it will just chip off in stead of shaving off under heavy wear. This characteristic can be considered less pretty than the wear of a nitrocellulose coating.
Danish and ‘tru-oil’ oil
Danish oil, tung oil and tru-oil are all oils in the same category, but made by different manufacturers. Nevertheless, the working is similar. The oil is being rubbed on the piece and polymerizes with the air to form a film. Since the chemical reaction needed to dry the oil is an exothermic reaction (i.e.: forms its own heat) the rags used to wipe on the oil can spontaneously ignite if the rags aren”t laid out flat. In case the rags aren’t laid out flat, the generated heat simply cannot escape and may ignite the oil. The oil is by no means as combustible as nitrocellulose. Nevertheless, it never hurts to take any preventive measurements.
Polymerized oils are easy to apply by yourself since no special equipment is needed. The results can be considered stunning: a soft, satin glow with a natural feel.
The choice of finish is highly dependent on the results you are aiming for. I like some guitars to be finished so the end result is a high-gloss guitar, but other guitars simply need a simple, natural oil finish to highlight the natural beauty of the woods.