Guitar Lacquers, How they affect your instrument
The lacquer on your your guitar is the layer on top of the paint, filler or stain of the wood. It is what protects your guitar from the outside world where moisture or the weak acid in your sweat may damage or warp the wood, it also allows your guitar to be buffed to that nice shine. Hence this layer is very important.
There are many different lacquers used by various companies to coat guitars. The main 3 types are Nitrocellulose, Polyester and Polyurethane. These all have their own different characteristics and behaviors which are important to understand when comparing guitars.
Nitrocellulose (Commonly shortened to Nitro) is one of the oldest guitar lacquers that is still in common use. It was invented in the late 1800's and was often first used in film however it was then also found do be a very good lacquer. Nitro was used heavily in the automotive industry in the 1930's-50's and its use in the guitars rose from there when Leo Fender (Owner of Fender guitars) saw its suitability for coating his first solid body guitars. As the popularity of electric guitars grew most other companies started used nitro as well. Nitro never fully sets, it is still evaporating slightly all of the time. This has an advantage when painting that you can apply many very very thin coats and they will all merge together, this means you can get a full and consistent coating that is very thin. Nitro also wears very easily by gradually dulling, yellowing and creating thin cracks called "Checking" if exposed to sudden temperature change. Some people like this wear and "relicing" some people don't.
There were issues however with Nitro, is that it is very hazardous to work with as it is very highly flammable and poisonous to inhale, thus there are now many regulations on its use. Due to this (and the fact that Nitro takes more time to apply) many companies have replaced using nitro with other options. All Gibson USA guitars however are still coated with Nitrocellulose.
Polyester and Polyurethane
Both of these finished are similar due to the fact that they are both "plastics" and made from hydrocarbons. This means that act a lot more conventionally then Nitro by setting to a solid finish around the guitar, this means that unlike nitro then you have to put that whole coat on in one go, you can't "build it up". Neither is as hazardous as nitro.
Polyester (Poly) is a more viscous solution than polyurethane and drys very quickly. This means it is normally applied in very thick coats. If you drop a guitar with a Poly Lacquer then the finish shows relatively little damage however is likely to chip and sometimes crack around that area.
Polyurethane (Urethane) is quite thin before it is applied and can be almost wiped onto the guitar, this allows the finish to be quite thin (Not as thin as nitro however). When impacted this finish is less likely to chip like poly does however tends to create larger more spread out cracks.
Due to the plastic nature of these lacquers they do not age like nitro. Apart from scratches and chips they keep the same colour over time without yellowing and do not dull.
The most common argument over lacquers is "do they effect the tone". Some people will swear that they do, whilst others will say that they cannot hear a difference. Nitro is reputed to have a better "tone" this is because the finish is very thin and thus allows the wood to resonate more easily at the woods natural frequency, thus improving sustain. This does follow logic because if you coat the guitar in a very thick layer of effectively plastic then you wouldn't expect it is vibrate how it did before. Urethane has a thinner lacquer however is also sometimes critisised for having bad "tone".
Another reason why guitars with a nitro lacquer may sound better is because typically only more expensive guitars are finished in nitro, and thus you would expect these more expensive guitars to sound better than cheaper poly/urethane finished guitars.
In conclusion whilst nitro will make a difference to the sound of your guitar due to the thinner layer it is questionable how audible this difference in tone actually is.