The term "urethane" is somewhat of a misnomer when used to describe a paintable protective coating. Urethane, also known as ethyl carbamte, is an organic chemical that occurs naturally in fermenting alcoholic beverages and was first produced artificially in the nineteenth century for medicinal purposes. Researchers discovered that stringing together chains of urethane molecules by a polymerization process produces a stiff, hard compound that has many industrial applications. Clear finishes for wood and metal often contain this compound, which is more properly known as polyurethane. The different types of this clear finish depend primarily on the solvent in which the polyurethane is dissolved — typically water- or oil-based substrate — and the process by which the finish cures, or hardens.
A clear urethane finish does not penetrate wood as readily as shellac or lacquer. The transparent coating remains on the surface and cures as the solvent in which it is dissolved evaporates. Some products cure when chemicals in the original mix react with moisture in the air, others cure when the chemicals react with each other, and still others are hybrids, hardening to the touch when the solvent dissolves and continuing to harden over a period of days. After an application of clear urethane hardens, another can be applied over it. Building up coats in this way is the usual method to achieve a thick, smooth coating on wood.
The drying time and workability of a clear urethane finish is determined to a large extent on the solvent in which it is dissolved. Clear urethane for use on cabinetry, woodwork and hardwood floors can be carried in a water- or oil-based substrate and is usually a hybrid, taking several days to fully cure. Water-borne urethane finishes typically take less time to harden to the touch than oil-based ones and don't give off toxic fumes, making them safer to use.
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The solvent that carries a clear urethane metal finish is usually highly volatile so that it will evaporate quickly and leave the surface ready for sanding and re-coating in a short time. This kind of urethane has to be sprayed, and a typical automotive paint job typically involves several stages of spraying and sanding. Automotive refinishers can avoid painting successive coats by using a single-stage urethane finish. This type of product comes with a hardener that catalyzes a reaction in the urethane to cure it, creating a harder and more protective coating than one that cures by exposure to the air. When properly mixed, single-stage urethane normally dries to a finish hard enough to sand in 24 hours, although it might take longer to dry if insufficient hardener is added.