This is a liquid plastic resin that was developed in the 1950s. It is very tough and durable, which is why many guitar manufacturers began using it. During the curing (drying) process, a chemical reaction takes place with oxygen and this causes the polymers in the finish to cross link and bond together very strongly. Because of environmental issues, water-based formulas are beginning to appear, replacing the solvent-based finishes. Water-based polyurethane does not cause any changes to the color of wood, while solvent-based finishes do cause some light-colored woods to have a slight amber tint.