There is a lot of debate in the guitar world amongst players, builders and manufacturers about the best finish for guitars. Not only that but there are (and always have been) fantastic marketing descriptions for the finish and why it's better for "tone". I've heard many people talk about how lacquer "lets the wood breathe" as just one example. The last time I checked wood doesn't have a respiratory system so what on earth are they talking about? Perhaps they mean the wood resonates "better" or "easier" or maybe they're referring how wood will absorb and release moisture from the air.
Let me back up for a second...the "best" finish for guitars doesn't actually exist. What is best for one guitar builder might not be best for another builder and what works best for any particular production facility will vary too. There are a wide variety of considerations, a wide variety of finishes and even more formulations within any given type of finish.
Historically shellac was widely used as a finish on musical instruments and is often described in mystical or romantic terms. Shellac has been widely used as a finish for hundreds of years on all sorts of wooden objects and in many respects shellac makes a wonderful finish today. Once nitrocellulose lacquer was introduced commercially in the early 1930's shellac started to lose ground as one of the most popular wood finishes. However one of the main resins used in early nitrocellulose lacquer was shellac so early lacquer formulations where very similar to shellac in appearance. It's important to note that during this time guitar building was primarily an industrial undertaking. There weren't a wide variety of individual guitar makers until much later in the century. As trends changed in other industries, finishes would evolve and companies making instruments followed suit. Nitrocellulose was abandoned by the auto industry by the 1970's and gained a bad reputation overeall because of its high volume of solvents referred to as "volatile organic compounds". However the guitar industry has to a significant degree stuck with lacquer and it is still being used today.
By the 1950's other lacquer formulations were being explored to address some of the shortcomings of nitrocellulose. Cellulose acetate butyrate or "CAB" lacquers began appearing and the automobile industry was quick to make the change because nitro lacquer and enamels colors of that time were quick to fade and in addition, lacquer was brittle so the finish quickly cracked. CAB lacquers were "water white" or completely clear as opposed to the amber nature of nitrocellulose based lacquers so color reproduction was easier and color retention was better. Sometimes CAB lacquers are referred to as acrylic lacquers but remember, any type of finish will have many different formulations custom tailored for specific applications and the addition of acrylic resins gave CAB lacquer added durability.
As time transitioned into the late 1960's more finishes were gaining popularity and by the early 1970's polyester finishes were being used. There are some understandable characteristics of polyester that make it appealing for production settings beyond just durability. These finishes have a very high solids content so they take fewer coats to build a film on the surface. They are catalyzed which creates a chemical reaction causing the molecules to cross-link making the finish incredibly tough. If you consider building thousands of guitars per year then higher solids, fewer coats and faster cure all translate into higher profits. It makes "cents".
As the auto industry continued to change the guitar coatings would evolve with it. By the 1980's two-component urethanes were readily available and are appealing because they share some of the working characteristics of lacquer with much of the durability polyester has. The term "poly" is used frequently in guitar circles as a catch-all type of term. Polyester and polyurethane fall under the umbrella of "poly" but they are actually very different. I won't bore with you technical mumbo-jumbo but hopefully you'll take a leap of faith and trust me for now.
The "Best" Choice
As I said earlier the "best" finishes for musical instruments does not exist but opinions most certainly do. I'll offer up some basic information on characteristics people consider when choosing a wood finish.
- Ease of use
- How "forgiving" is the finish to use?
- How is the finish applied?
- What sort of precautions or facilities are necessary to apply the finish?
- How many coats will it take to build the finish?
- How much time does it take to apply the finish?
- How long does it take the finish to "cure"?
- How thick will the finish be?
- How flexible is the finish?
- How hard or durable is the finish?
- How easily could the coating be damaged?
- Does it have a "warm" appearance"?
- Does it look "washed out" or "cold" on the wood?
- Does it look "plastic"?
- If and when the finish is damaged, what can be done to fix it?
Sound / "Tone"
Here's where the case for one type of finish over another gets confusing, overstated and hyped. While there are many valid opinions about which type of finish sounds "best" I'll try to cover steer away from the hype and focus instead on finish characteristics.
First things First
Nearly every finish on a musical instruments will build to a film or thickness on the surface of the wood. How thick the finish is probably has the most to do with how "good" the finish "sounds". If you are relying on wood to vibrate then less is more right? The reality is, different types of finishes either have a physical limit to how thick they can be applied or they don't. Shellac, spirit varnishes, nitrocellulose and CAB lacquers all have a "dry film limit". That means, a limit to how thick they can be built on the surface before they will fail prematurely (crack). So after all is said and done the finish on a guitar with nitrocellulose will be around .003" thick on vintage guitars and perhaps in the range of .005"-.007" thick on a modern lacquer finish. If lacquer is applied more than .007" the likelihood it fails increases to almost certainty.
Modern catalyzed finishes like 2K urethane and polyester have no limit to how thick they can be applied. In some production settings that means the finish can be applied very thick and sanded down smooth without worrying about cutting through the finish back to the wood. The final result is these finishes can easily be .030" - .060" thick or even more! By the time you have nearly 1/16" of finish on the wood it must start to inhibit vibration right?
This is NOT to say any company or any person using a catalyzed finish is a doing a cheap or sloppy job of finishing. These modern finishes have the benefit of fast dry times, chemical curing processes (shorter cure times) and can be applied in reasonable film thickness to yield beautiful results. I heard or read comments from a guitar builder who said something like "it isn't what is used, but how it's used". I'm paraphrasing and unfortunately can't remember who described it that way but I agree with them.
People love to rave about nitro finishes "just like the old days" or "classic tone" that sounds better. There is a certain mojo about vintage instruments but the finish is just one part of the puzzle. The reality is nitrocellulose was the standard industrial finish from the mid 1930's through the 1960's. It tends to be a more brittle finish than modern catalyzed coatings and therefore has plasticizers added to make it more flexible. Even a skillfully applied lacquer finish will likely crack with age based on temperature and humidity changes as the plasticizers eventually disappear. After about 30 years the all the plasticizer is gone and the finish is very, very brittle.
Personally I prefer nitrocellulose lacquer because it's very forgiving to use, has an amber tint which gives depth and warmth to the wood and what I consider to be a "traditional" guitar finish. Lacquer has less solid content and therefore requires more coats to build to a suitable thickness on the wood. Less solid content means more solvent content and that is how lacquer dries. The solvents evaporate and leave just the solids on the surface of the wood. There is dry time between coats, reasonable limits on how many coats can be applied in a day and many days or weeks of dry time to let enough solvent evaporate to a point where the instrument can be sanded and buffed.
For the average player all of this information probably isn't that important. If you find an instrument that plays well, looks good and speaks to you on some level you'll enjoy playing it and at the end of the day, that's what really matters. Some people shopping for a guitar might be turned off by any appearance of grain lines in the top showing through the finish. When I see that I think to myself "ah, a nice thin finish" which I like much more than a thick/caked on polyester finish which looks encased in plastic. As with many things it usually comes down to personal preference and if the guitar plays and sounds good to you then it is good.