On Applying Polyurethane
What do an oak door, a walnut mantle, and a cherry rocker have in common? Aside from the fact that each is constructed of natural wood, they 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. The decision to one or the other rests primarily with the desired look and how the object will be used. For example, antique cherry rocker will look stunning (and last ions) 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. Keep in mind that wood and water don’t mix. Thus, 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 and longboats. More recently, shellac – made from the shells or small scale 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 should never 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 great for bar tops, tabletops, doors, hardwood flooring, etc. Although there are water-base urethanes, we are a bit old-fashioned and prefer oil-base polyurethane because of the way it flows and the super hard finish that it achieves. We are often asked if polyurethane can be used on exterior surfaces such as siding, decking, and patio furniture. Our answer is invariably NO. When exposed to lots of 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 too can have professional-looking results. Begin by making sure that 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 of the sanding dust and wipe the surface with a clean cloth dampened with denatured alcohol.
If you will be staining your object, now is the time to apply the stain. Wipe on the stain using a soft, clean cloth. Apply the stain as evenly as possible being careful to remove any 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, it’s a good idea to 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. Yikes!
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 of 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 will usually require three to five coats.
After the final coat has completely dried, 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 – just 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 very 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 temperature.
- Work in a dry, dust-free and well-ventilated space. Plastic drop cloths hung from the ceiling can create the perfect work environment.
- Don’t use old material. Over time the solvents evaporate and alter the composition of the material. Use fresh material whenever possible.
- 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 to back it up.
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