Finish is a term used in woodworking for the protective film or coat, either wiped, brushed, or sprayed on the surface of wood, in this case. In our case, a stringed instrument. A question sometimes asked is whether a “finish” coating really necessary? Would a stringed instrument sound better if it had no finish?
With absolutely no material coating on the wood, it’s true that there would be no hindrance to tone and volume. However, some builders believe that a professionally applied finish focuses and evens out tone and volume.
But regardless of tone, the fact remains that all wood, whether made into furniture, floors, walls, or stringed instruments needs protection. Protection is necessary due to the nature of wood. All living things have cells or cell structure, and although technically not still living, this cell structure or chambers in the wood are open pores that extend to the surface. These open chambers are like tubes, still “breathing” or sometimes called transpiring. These open pores are still absorbing and releasing moisture, depending upon the amount of humidity in the environment.
Besides moisture, this porous surface is subject to human perspiration, body oils, dirt, foods, and alcohol. And thus, some form of protection is necessary. In the musical instrument industry, this protective coat is usually applied in various forms of oils, varnishes, shellacs, and most often today, modern formulations of lacquer. Lacquer is an old term for a material used as a coating. It comes from the word lac insect because the secretions of particular insects were original used to formulate Shellac. But now, modern chemistry has enabled the finishing industry to manufacture a more durable coating, and the name is simply “lacquer.”
Modern “lacquer” can be either Gloss or Satin. The formulation is similar, but the technique of application enables it to either be a high sheen (gloss), or matte (satin) in appearance. Another “lacquer” coating, though often not considered a lacquer is Polyurethane. This finish is also a modern formulation, producing a very durable coating. The term lacquer today usually applies to what is known as Nitrocellulose. One difference in Nitrocellulose lacquer and Polyurethane is durability. Polyurethane solidifies much harder and more protective. Another difference in the two is the amount of solids verses solvents. Nitrocellulose lacquer is approximately 25% solids and 75% solvents. In other words, after being sprayed and dry, 75% was solvent, and thus evaporated.
Whereas Polyurethane is the opposite, with 75% solids, and 25% solvents. So, most of what is sprayed on remains. Which is good, as long as whoever is applying the finish knows how to apply thin coats, otherwise the guitar or ukulele will look and feel like it was dipped in a bucket of honey. And if not applied correctly, the tone and volume will be poor quality. So again, if done properly, both finishes are good. Another difference, which is important is the ability to repair the cured finish if necessary. Nitrocellulose lacquer is easily dissolved or sanded, and then subsequent coatings will will “melt in” and adhere to the base coats, and then can be final wet sanded and buffed to look almost perfect. However Polyurethane is hard and difficult to repair. There are other finish coatings we have used through the years. Water based lacquers, natural tree oils, and french polish shellac.