I read this article entitled "Finishing touches" by Dana Bourgeois published September 2004 and found it most interesting. Source:
http://www.musicplayer.com/article/finishing-touches/Sep-04/3214
Several years ago, I started finishing acoustic guitars using catalyzed urethane over polyester primer. When word got out that I no longer used traditional nitrocellulose lacquer, Internet guitar forums twittered at the notion that I had switched to “plastic” finishes.
Wait a minute! Isn’t nitrocellulose the same material used to make “plastic” bindings and pickguards? Curious about the definition of plastic, I consulted Merriam-Webster’s online dictionary, and found this description: “Plastic (n): …organic synthetic or processed materials that are mostly thermoplastic or thermosetting polymers of high molecular weight and can be made into objects, films, or filaments.”
All modern finishes are films of thermoplastic or thermosetting polymers of high molecular weight. Some, like nitrocellulose, cure by evaporation of solvents, and others cure by a process of catalyzation. All, nevertheless, fit Merriam-Webster’s definition of plastic. It is a misconception, then, to think that urethane and polyester are somehow more “plastic” than nitrocellulose lacquer. Ironically, nitrocellulose lacquer is arguably more “thermoplastic” than urethane and polyester, because of its tendency to become reamalgamated, reformed, or reconstituted with heat, solvents, and the passage of time.
On a molecular level, nitrocellulose lacquer can take years to fully crystallize. The process of crystallization—commonly known as curing—contributes significantly to improvements in a guitar’s volume, clarity, and harmonic range. Where lacquer takes years to fully cure, catalyzed finishes such as polyester and urethane harden, and fully crystallize, in a matter of days. Though all finishes have some damping effect on acoustic soundboards, it is not true that all guitar tops would benefit from being left unfinished. Finish makes up a significant portion of a soundboard’s mass (the principal measure of inertia) relative to its volume (the amount of space occupied—not the kind measured in decibels). To the extent that mass plays an important role in tone production, the portion of mass contributed by finish is critical. Manipulation of finish mass—along with top graduation, brace layout, and brace shaping—contributes to the mechanical design by which a luthier’s unique tonal signature is defined. Within reasonable parameters, the important thing is not how heavy, how thick, or how hard a finish is, but how it incorporates into an integrated soundboard design. The reason I switched to catalyzed finishes was because relatively inert substances are inherently more controllable than substances whose properties are constantly changing.
When I started using catalyzed finishes, I noticed my guitars sounded a bit more “broken in” at an earlier age. Ricky Skaggs once surprised both of us by using one of my guitars on a recording only 20 minutes after unpacking it from a shipping carton. Bryan Sutton recorded most of Bluegrass Guitar on a three-week-old Bourgeois dreadnought. Incidents like these suggest that guitars ought to be judged by the music that can be made on them, rather than by the presence or absence of particular components. Design details, material selection, construction method, etc. are, at best, only arbitrary indicators of quality. At the end of the day, these factors are relatively insignificant in comparison to tone, playability, durability, and aesthetic considerations.
