Polyurethane was invented in Germany in 1937. A highly versatile plastic, it was introduced into U.S. commercial manufacturing in the 1950s. In liquid plastic resin form, it made an extremely fast-drying, tough and durable finish (also used for dance floors and bowling alleys) that formed a much harder and abrasion-resistant layer around a guitar than nitro, while still allowing good wood resonance. Further, urethane ages well—it doesn’t check, crack and yellow as nitro finishes do. If you aren’t a fan of the beat-up look, a urethane-finish instrument is probably for you.