In the guitar world, there are two major categories of chemistries used to finish electric and acoustic guitars. One is the traditional “Nitro” nitrocellulose finish, and one is the modern “Poly” or polyurethane finish.
Early guitars used the Nitro style finish because that was the available chemistry of the day for both classical instruments and electric guitars. Many of the techniques for making classical instruments transferred directly to electric guitars, especially the traditional set-neck hollow body models.
Nitro finishes have many desirable qualities: They are thin, repairable, flexible, and don’t inhibit the vibrations of the wood. For a purely acoustic instrument these are highly desirable qualities, and a bad finish will kill the tone of an acoustic instrument. Nitro finishes are also labor intensive to apply, slow, are highly flammable, and toxic. They are a VOC (volatile organic compound) and subject to environmental limitations. Special permits are required to have a spray booth that uses lacquer finishes, which also raises the cost.
From a pure standpoint of manufacturing efficiency and cost, the more modern Poly finishes are the dominant method. They cure using a catalyst, which can be accelerated with heat or UV light, and compared to a Nitro finish, can be applied in a fraction of the time. They are also extremely glossy, durable, and don’t tend to crack or check over time.
The wrap on a Poly finish is that they are thick and inflexible, and don’t allow the instrument to vibrate and resonate as well as a lacquer finish. This is completely possible, but there are many variables in how the finish is applied, the amount of wood fillers, undercoats, etc. Environmentally conscious companies such as Taylor have put a lot of research into thin, flexible modern coatings that provide great sound quality but are safe for the environment and their employees.
Lacquer finishes by their very nature are a good match for instruments, but their cost makes them viable only on higher end products. So are Nitro guitars better sounding because of the finish, or better sounding because they are only applied to guitars of higher overall quality? It’s probably a little of both, and because of that it’s hard to separate the two.
Some players will only play Nitro guitars, and if you have the means to own that level of instrument, you’re unlikely to be disappointed. However, a guitar is always the sum of its parts and workmanship, and finish is just one of the components. Nitro guitars make up a tiny fraction of the market, so obviously there are scores of wonderful guitars made every year utilizing modern finish chemistries. The finishing method is an indicator, but always let your ears be the guide.