What's the difference between nitro and poly finishes?

Depends who you ask. Ask a vintage guitar snob and they'll tell you nitro is alive, poly is plastic, and never the twain shall meet. Ask a factory that ships ten thousand guitars a month and they'll tell you poly is consistent, durable, and doesn't require a padded room and a prayer to apply. Both are right. Neither is telling you the whole story.

Nitrocellulose lacquer

Nitro is thin. Really thin. A few thousandths of an inch once cured, which is part of why guitars finished in nitro are often described as resonating more freely — there's simply less mass and stiffness sitting on top of the wood, muting it. It also ages. It yellows, it checks (those little spiderweb cracks on old guitars), and it wears through at contact points over decades of play.

The catch: nitro is soft and reactive. It's sensitive to heat, prone to checking with temperature swings, and doesn't play well with certain rubbers, plastics, and solvents. Leave a rubber-backed capo case against it for a summer and you might get a permanent souvenir.

Polyurethane / polyester

Poly is thick, tough, and chemically stable. It cures hard and shrugs off contact that would leave nitro with a mark. It doesn't check or yellow the way nitro does, which is exactly why it doesn't develop that "vintage" patina people get sentimental about.

Whether poly "chokes" resonance is a real argument among real luthiers, and I'm not settling it here. What I will say: a poly finish is not the reason your guitar doesn't sound like a 1950s Martin. Wood selection, bracing, and construction quality matter enormously more than finish type.

So which one do you have, and does it matter?

For maintenance, yes — poly tolerates more, nitro needs more care. For tone, it matters less than the internet wants you to believe. If you're not sure what you're playing, treat it like nitro. Worst case, you were more careful than you needed to be.