Popular Woodworking 2007-02 № 160, страница 68
Flexner on Finishing Test to Find a Durable Finish A few sample boards can save you from future finishing headaches. Finish manufacturers targeting the furniture industry and large professional finish shops provide a lot of information about their products. Unfortunately, the manufacturers who target amateur woodworkers and small shop professionals aren't as helpful. In the end, we are often left to our own devices to figure out the characteristics of the finishes we are buying. Here are some easy tests you can perform in your shop to determine the qualities of the finishes you're using. In most cases you'll want to do the tests on scrap wood. For the most accurate results, let the finish cure at least a couple weeks in a warm room before performing the tests. Scratch Resistance The ability of a finish to resist damage from coarse or sharp objects is likely one of the most important qualities about which you're concerned. To a large extent you can know the comparative scratch resistance simply from the type of product. For example, oil-based varnishes (including polyurethane varnish) and catalyzed lacquers are much more scratch resistant than shellac and nitrocellulose lacquer, and a little more scratch resistant than acrylic and polyurethane water-based finishes. But what about differences among brands within each finish type? Or what about the comparative difference between a water-based polyurethane and an oil-based polyurethane by Bob Flexner Bob is the author of "Understanding Wood Finishing" and a contributing editor to Popular Woodworking. To determ ine the relative scratch resistance of a finish, push the sharp edge of the cylindrical lead in various "B" and "H" architect's pencils forward on a finish. There's no need to use pressure. The next-to-hardest pencil that doesn't scratch the finish is the rating for that finish. - that is, how much scratch resistance are you actually giving up by using a water-based polyurethane? To determine a finish's scratch resistance, purchase a set of architect's drawing pencils ranging in hardness from about 2B (soft) to 5H (hard). Sharpen each pencil using a knife so you leave the sharp cylindrical edge of the lead intact. If you damage this edge, or if it becomes worn, sand it flat, holding the pencil 90° to the sandpaper. Beginning with one of the softer pencils, hold it as you would for writing and push it forward across the cured finish. Maintaining equal pressure, follow with pencils of increasing hardness until you find one that cuts into the finish. The hardness rating for that finish is the rating of the previous pencil - the hardest lead that doesn't cut. Water Resistance Another quality you may be looking for in a finish is water resistance. As I explained in my article "The Thick and Thin of Wood Finishing" (December 2006, Issue #159), a thickly applied finish of any type is very water resistant. So testing won't be very revealing except on thin finishes such as oil or wax, or any film-building finish applied with just one or two coats. To test for water resistance, make up a sample board with the same number of coats applied in the same manner as on your project. Then place a small puddle of water on the surface and cover it with a small metal or glass cup or jar to prevent evaporation. Check % Br * ' 7 The relative water resistance of a thin finish can be determined by placing a small puddle of water on the finish and covering it to prevent evaporation. Check at regular intervals for damage. 100 Popular Woodworking February 2007 |
