What are the pros and cons of urethane coating?

A can of polyurethane.

Even though a urethane coating provides superior protection on wood and metal, it is not always the best choice for a topcoat. Urethane, or more appropriately, polyurethane, is an artificial polymer and, although its impact strength surpasses that of varnish and shellac, it does not penetrate wood in the same way and remains on the surface. A urethane coating will level well on metal and will adhere to any previous finish, but its plastic appearance can be unsightly. While typically affordable, urethane is not the cheapest finishing material available.

A disadvantage of urethane coatings is their inability to penetrate the wood below the surface.

While lacquer and shellac, which are natural polymers, harden by evaporation of the solvent in which they are transported, a urethane coating needs to cure. As the solvent evaporates, polymers form by reacting with chemicals in the paint or with moisture in the air. The final finish is more resilient and durable than one created by natural polymers, but the waiting period between successive coats is longer. Depending on the curing process, this period can be 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 coating is much less vulnerable to staining or discoloration than other coatings, making it a better choice than lacquer for countertops and bars, where spills are frequent. It has a higher impact resistance, so it is also less likely to chip under the abuse these surfaces undergo. The higher chemical and impact resistance makes urethane an effective automotive finish, but applying it correctly requires care because it doesn’t flow out of the spray gun as easily as lacquer or enamel. Additionally, the toxicity of automotive urethane ingredients requires the use of expensive protective clothing, eyewear, and a respirator connected to an air pump.

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Paintable urethane products for use on wood are available in both oil-based and water-based varieties. In contrast to urethane products for automotive use, the 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 appearance of a urethane coating can detract from the appearance of certain wood varieties, especially dense exotic hardwoods such as teak and mahogany. These woods may look best if left unfinished or simply coated with oil.

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