Believe it or not, guitars breathe, and they do need to breathe if they are to sound good. Take out a guitar that has been kept in its case for ages, especially an acoustic guitar, and pluck the strings. It will sound muffled and stifled. The ringing sustain with clear overtones that you expect from a good guitar will be absent. But leave the guitar outside its case for a few days, in a place that's not so humid, and there will be a difference in the sound. Good wines breathe, and so do guitars. The type of finish (paint) used on a guitar is a very important factor in its ability to breathe. Many vintage guitars sound great because they do not have thick coats of paint but were au naturel, or had only a thin coat of paint followed by a top coat of nitrocellulose lacquer. The ban on nitrocellulose lacquers (for health or environmental reasons I think) and the fashion for guitars to have custom colors like Shell Pink, Pacific Blue, Candy Apple Red, etc resulted in guitars that could not breathe.
Many guitars stripped of their paintwork become more resonant with clear ringing sustain. Of course, many other factors are responsible for a guitar's innate tone including:
1. The wood: Some say the swamp ash and maple in days of yore were of better quality, and the wood was also alllowed to dry out slower and longer in a kiln.
2. The particular combination of bridge, the nut, type of wood used in the neck and fretboard, angle of tilt in headstock, type of joint from neck to body, and nearly everything else.
3. How long and how much the guitar has been played. It seems that the vibrations generated by playing cause changes in the molecular structure of the guitar's wood. Some studies done after subjecting a guitar to 40000 hours of vibrations on some machine, to simulate hours of playing, do support this theory.
Not all expensive guitars sound good, and some cheap guitars do sound good. The picture above shows my limited edition ( one of 20 copies made) Fender Telecaster. It is a collector's item, very well put together, and a joy to behold. The pickups are of the best quality, and yet the sound is not inspiring at all, which is why I just can't bring myself to play it. The thick Burgundy colored paintwork has stifled the guitar's 'voice'. And so it will remain unplayed and unloved.