Painting the House

Polyurethanes: A Definition

Polyurethane coatings were invented in the mid-20th century. Plastic resins in liquid form, they are used primarily to protect wood surfaces. Their main ingredients include acids derived from vegetable oil, nitrogen-based chemical amalgams, isocyanates, and mineral spirits. Nitrogen helps keep polyurethane finishes from chipping, isocyanates make them hardier in general, and mineral spirits constitute the base that supports the other components.

A polyurethane coating cures when some of its chemical elements form powerful links with one another after reacting with atmospheric oxygen. These links allow the finish to resist damage due to external factors. In contrast, lacquer does not stand up well to strong chemical agents. However, polyurethane is prone to eventual discoloration, and its application may make light woods darker.

Water-Based Polyurethane Products

Ecological awareness has prompted several finish manufacturers to make water-based polyurethane products. The basic formulation calls for the suspension of polyurethane in water. Although these coatings are more vulnerable to wear and tear than traditional, oil-based finishes, they dry rapidly, produce almost no smell, and are water washable. They also represent less of a health hazard than oil-based polyurethanes, which should only be applied in a properly ventilated area.

Oil-Based Polyurethane Products

Oil-based polyurethane cures most efficiently when the temperature is neither too high nor too low. Not surprisingly, humidity level is the key factor when a water-based finish is curing. On average, it is necessary to apply one more coat of water-based polyurethane than of oil-based finish. Whatever the nature of the coating, sanding and cleaning the wood after each layer has dried is advisable. Oil-based coatings are inflammable and more environmentally harmful than water-based alternatives, which do not burn or yellow and can be washed with water and simple cleaning products. The wait between applying one coat of oil-based finish and adding the next may be as short as four hours or as long as 18.

How to Apply Polyurethanes

Under no circumstances should one shake a polyurethane finish prior to applying it. Agitation creates bubbles, which may later appear on the surface being treated. Gentle stirring is the recommended way to mix polyurethane. A cloth, foam brush, or fine-bristled brush may be used to apply it to a surface. When brushing, follow the wood’s grain and get rid of every bubble you see.