Election Section

The art of applying polyurethane

By James and Morris Carey, The Associated Press
Friday November 30, 2001

What do an oak door, walnut mantle and a cherry rocker have in common? 

Besides being constructed of natural wood, all look especially good with a clear finish. In fact, most natural woodwork jobs call for one or more topcoats of a clear finish. That’s not to suggest that they can’t be stained – they can. 

An antique cherry rocker will look stunning, and last for a very long time, with a hand-rubbed oil stain. The oil stain will feed the wood fibers – keeping them moist and supple. On the other hand, an old oak table that is used primarily for daily dining will last longer, look better and be easier to keep clean with a hard, clear finish. Wood and water do not mix. If the object will be exposed to water from time to time, go for a clear finish. 

Nordic seamen used boiled linseed oil as a coating to protect their ships. More recently, shellac – made from the shells of small insects dissolved in alcohol — was popular as an interior wood finish. Shellac is fast drying, but it is fragile and has a tendency to yellow in sunlight. It is also highly flammable. It never should be applied in the presence of an open flame, including a pilot light. 

Our favorite clear finish is oil-base polyurethane. Essentially an oil-base paint without pigment, polyurethane is an extremely durable clear plastic finish that is ideal for bar tops, tabletops, doors, hardwood flooring, etc. Although there are water-base urethanes, we prefer oil-base polyurethane because of the way it flows and the super-hard finish that it achieves. 

We often are asked if polyurethane can be used on exterior surfaces such as siding, decking, and patio furniture. Our answer is no. When exposed to much sunlight, a clear finish will crack, bubble up and rapidly deteriorate. A penetrating oil finish (clear or a pigmented stain) is a better choice for exterior use. 

Polyurethane can be used successfully to finish an exterior door provided that it has reasonable protection from the elements. A better bet for an exterior door (or other exterior objects that might be appropriate for a hard finish) is a marine spar varnish. Spar varnish is a tougher, more durable finish that will hold up better to prolonged exposure to sunlight and water. However, it, too, will require regular upkeep. 

You don’t have to be a pro to have a finish that looks like glass. With a little patience and the right tools and materials, you can have professional-looking results. Begin by making sure the surface is clean, dry and smooth. Sand the surface with progressively finer paper beginning with medium (100-grit) sandpaper, then 150-grit and finish with 220-grit. Use a vacuum cleaner with an upholstery brush, along with a tack cloth to remove all the sanding dust. Then wipe the surface with a clean cloth dampened with denatured alcohol. 

If you will be staining the object, now is the time to apply the stain. Wipe on the stain using a soft, clean cloth. Apply evenly, being careful to remove excess. Allow the stain to soak in overnight before applying the first coat of polyurethane. Since heavily grained wood – such as oak – absorbs stain unevenly, first apply a clear sealer. Some stains are self-sealing and, thus, don’t require sanding sealer. The label directions should specify what you are using. 

If you will be skipping the stain, the first coat of clear finish should be thinned as follows – three parts polyurethane and two parts mineral spirits. This will help the polyurethane penetrate the wood more easily and reduce the number of brush marks. The thinned polyurethane should be brushed over the entire surface to be finished. For best results, use a Chinese (natural) bristle brush designed for use with oil-base stains and paints. Synthetic brushes will leave brush marks. 

After the initial coat has dried (usually overnight), lightly sand it smooth using 220-grit sandpaper. Use the vacuum and upholstery brush along with a tack cloth to remove all the dust, and prepare for the next coat. Unlike the first coat, the second coat of polyurethane doesn’t need to be thinned. In fact, if the material is too thin, or if too much is applied, it will usually run – which can be quite a chore to repair. If a run does occur, allow it to dry and cut it away using a razor blade. Be careful not to cut into the surrounding finish. Small drips will disappear when the finish is wet-sanded. 

Repeat the process for the third coat and any subsequent coats – making sure to sand between coats using a wet 400-grit wet/dry sandpaper to remove imperfections in the surface. Two to three coats should be sufficient for a previously finished surface.  

New wood usually will require three to five coats. 

After the final coat has dried completely, use an automotive rubbing compound first, and then a polishing compound to remove minor scratches and to achieve a glasslike finish. Finish the job by buffing the surface with a clean cotton cloth. 

Here are a few tricks that will make for professional results: 

—Don’t shake or vigorously stir a can of polyurethane. Doing so causes the material to bubble in the can and on your work. The material should be stirred, but not vigorously. 

—Always maintain a “wet edge.” Don’t allow the material to dry or it will become difficult to spread, and brush marks will become quite visible. Work quickly and without stopping, for best results. 

—Don’t apply finish when the temperature is either too cold or too hot. Check the label for best application temperatures. 

—Work in a dry, dust-free and well-ventilated space. Plastic drop cloths hung from the ceiling create an ideal work environment. 

—Don’t use old material. Over time the solvents evaporate and alter the composition of the material. Use fresh material. 

—Don’t apply polyurethane with a roller. It can be applied with a sprayer — but only if you have the proper equipment and the experience. 

For more home-improvement tips and information, visit our Web site at www.onthehouse.com. 


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