Urethane is a man-made polymer that provides superior protection on wood and metal, but its plastic appearance can be unsightly. It has higher chemical and impact resistance, making it suitable for automotive and countertop finishes. Urethane products for wood come in oil- and water-based varieties, with water-based being non-toxic. However, the plastic-like appearance may detract from the appearance of some wood varieties.
While a urethane finish provides superior protection on wood and metal, it’s not always the best option for a finish. Urethane, or more properly polyurethane, is a man-made polymer, and while its impact resistance exceeds that of lacquer and shellac, it does not penetrate the wood in the same way and remains on the surface. A urethane coating levels well on metal and adheres to any previous finish, but its plastic appearance can be unsightly. While typically affordable, urethane is not the least expensive finishing material available.
While lacquer and shellac, which are natural polymers, harden by evaporation of the solvent in which they are carried, a urethane coating must cure. As the solvent evaporates, polymers are formed by reacting with chemicals in the paint or moisture in the air. The final finish is stronger and more durable than that made with natural polymers, but the waiting time between successive coats is longer. Depending on the curing process, this period can vary from two hours to two days and, until curing is complete, the surface cannot be sanded and must be protected from dust.
Polyurethane polymer is designed to provide greater chemical resistance than natural polymers. For this reason, a urethane finish is much less vulnerable to staining or discoloration than other finishes, making it a better choice than countertop and bar paint, where spills are frequent. It has higher impact resistance, so it’s also less likely to chip under the abuse these surfaces take. Increased chemical and impact resistance make urethane an effective automotive finish, but applying it correctly requires care because it doesn’t come off the spray gun as easily as paint or enamel. Additionally, the toxicity of the ingredients in automotive urethane requires the use of expensive protective clothing, goggles, and a respirator attached to an air pump.
Paintable urethane products for use on wood come in oil- and water-based varieties. Unlike automotive urethane products, water-based polyurethane wood finish is non-toxic and the fumes it produces are safe to breathe. The excellent impact resistance and longevity of oil- or water-based polyurethane finishes make them suitable for floors, although the plastic-like appearance of a urethane finish may detract from the appearance of some wood varieties, particularly dense exotic hardwoods such as teak and mahogany. These woods might look better if left unfinished or simply covered in oil.
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