Site gibson : Incomprehension

There's no real benefit to nitrocellulose. It's an old finish that's stuck around because guitarists are conservative old farts. They want what was on the classic guitars of yesteryear, and nitrocellulose lacquer was the cheapest and easiest thing to put on them in the old days. It is easy to patch up for the same reason it doesn't have to have all that rubbing between coats and such. The solvent in the new coat dissolves the previous coats so it all ends up being one continuous coat. Actually, polyurethane has to be reapplied before the previous coat has cured if sanding's to be avoided, because it can't be dissolved after it sets and the new coat has to mechanically bond to it.

Nitrocellulose can best be thought of as a cheap synthetic substitute for shellac. It's the original manmade plastic, and was coming into popularity (and cheap wide availability) about the time mass produced cheap guitars came along. It saved the makers having to pay for a lot of brushing, hand sanding and rubbing. It is super easy to work with, and the only reason it's faded out of popularity in the US is that the EPA and OSHA have made it very difficult to use in compliance with restrictions on solvent vapors released in the air. That's not the case in all countries, and some cheap Asian made guitars come with nitrocellulose lacquer. Lacquer hardens by evaporation of the solvent, and lacquer thinner vapor's pretty noxious stuff.

It's not very resistant to water or chemicals like alcohol, and plasticisers in rubber and plastic (like those sleeves on guitar stands) will attack and mar it. It's easily stained. It doesn't resist ultraviolet well at all. It doesn't age well and shrinks and cracks. (Some folks like the look of that "alligatored" finish, though.) It also emits corrosive fumes that'll eat up your guitar's hardware in the case as it deteriorates. It has nothing to do with how your wood ages; it's bare on the inside, anyway!

Polyurethane's a superior finish in most respects, UNLESS you want to strip and refinish it, when its durability and resistance to solvent and chemical attack become barriers to removing the old finish.