Urethane paint is paint created by mixing pigment with polyurethanes, which act as the binder to hold the pigment molecules together. Many brands of urethane paint are available to consumers today. Semi-gloss urethane paints have been developed, but all urethane paint will have some degree of glossiness, because the same properties that make it resilient also make it reflect light.
Urethane paint was first created in the mid-1960s, utilising the newly-developed plastics technology of that era, to be superior to enamel paint in terms of durability. It also has the advantage of being a surface that can be applied over enamel if the enamel-painted surface is prepared first. Urethane painted surfaces are minimally affected by ultraviolet rays, making the paint a good choice for outdoor surfaces. It is widely used in the automotive industry because of its durability.
How it's made
Polyurethane is made of chains of organic units joined by urethane links. It is created when a monomer containing at least two isocyanate groups is reacted with another monomer containing alcohol groups. A simplified formula is: R1-N=C=O + R2-OH --->> R1-NH-COO-R2. Inexpensive urethane paint might contain as little as 10 per cent polyurethane. High-quality polyurethane paint has a higher percentage of the expensive polyurethane ingredient.
How to use
Because urethane paint is so resilient, it can be tricky getting it to stick to some surfaces, or even to the paintbrush bristles. Special brushes have been developed to hold and apply urethane paint. Priming the surface is important, especially sanding a smooth surface to give the urethane paint plenty of places to hold onto. The do-it-yourself painter can apply urethane paint in one of two ways. One is diluting the paint (with a special urethane thinner) and applying several thin layers, lightly sanding between each (fully dry) layer, until you build up an adequate coat of paint. Alternately, a thick coating of the paint can be applied with a small roller, then "tipped" by dragging the tips of an almost-dry paintbrush over the wet surface, which scores the paint and creates channels to allow the bubbles to escape.
Planning and Precautions
A urethane-painted surface can be expected to contract and shrink over time. This makes it bond even more tightly to an inflexible surface, such as the bonnet of a car. But painting over wood surfaces requires some forethought. When possible, it's best to paint both sides of the wood (such as the inside and outside of a door) with urethane paint to prevent the wood from warping as the painted surface contracts. Urethane paint is highly toxic in its wet, or uncured form, so DIY painters should use respirators specifically rated for this kind of work indoors. Exterior paints, like the popular Sears Weatherbeater urethane paints, should never be used indoors without a respirator.
Urethane paint can take a long time to dry or fully cure--up to three weeks in cold temperatures. The surface must be protected until it is completely cured. But if the paint has been correctly applied, once the surface has cured, urethane-painted surfaces are second only to powder-coated surfaces in terms of durability and should require little maintenance.