Polyurethanes, once considered ideal for finishing certain types of wooden floors, have developed into versatile coatings with a much wider range of practical uses. For one thing, they now serve equally well as furniture finishes.
The popularity of polyurethanes is due to several factors, even though they often cost more than most of our traditional finishes.
They are almost impervious to household chemicals, water rings and other stains. They are tough and resistant to scuffs, scrapes and everyday wear and tear. And, for clinchers, they look great.
With the introduction of satin and antique flat versions, polyurethanes now offer do-it-yourselfers a full range of sheens. Also, certain polyurethane formulas now contain inhibitors to protect them from the sun's ultraviolet rays, for exterior use.
Since the days when polyurethanes were considered an optimum coating for bowling alleys, which endured intense wear and required a finish with maximum impact and abrasion resistance, they have had the reputation of being a bit difficult to handle. This was especially so on projects requiring more than one coat.
John Molski, the technical director at United Gilsonite Laboratories, says the truth is that the successful application of a polyurethane finish is quite simple. The key is in the care taken during the preparation.
Most often, polyurethane clear finishes are applied to bare wood surfaces. They may also be applied to previously coated wood surfaces, but special care must be taken.
Existing finishes in poor condition must be stripped completely. Lacquer or shellac finishes should also be removed because polyurethanes do not adhere to them. Existing varnish finishes in good condition should be thoroughly sanded to remove all gloss. In any case, a smoothly sanded surface is essential to the success of any wood finishing project.
Many projects are stained to a desired color tone before final coating with a polyurethane. Before staining, work the surface from a medium (150) to a fine (220) grit sandpaper.
After the stain has been applied and given sufficient drying time, the surface can be prepared for coating with the polyurethane. Lightly sand the stained surface with 220 sandpaper. This will allow the polyurethane to better adhere to, or "grip," the wood surface.
After sanding is completed, vacuum or brush the surface clean, then use a tack cloth for a final wipe-down. If a tack cloth cannot be located in stores, make one by dampening a good quality cheesecloth in mineral spirits.
The polyurethane can now be applied. To ensure successful application, use at temperatures between 65 and 85 degrees Fahrenheit. If using a gloss polyurethane, do not stir; the coating is ready for application as is, Molski says. Stirring may introduce air bubbles into the coating that will adversely affect the finish.
Conversely, satin and flat polyurethanes contain a flattening agent that must be gently stirred into suspension--or, evenly reincorporated into the mixture. (Never mix any type polyurethane coating on a paint mixer.) Periodically, stir satin and flat polyurethanes during use, also.