![]() "The organic molecules can only handle so many photons before they fall apart," says Mark Knaebe, a chemist who researched paint formulas for 13 years at the Forest Products Laboratory. Clear coatings behave more like sunscreen: They contain chemicals that absorb UV radiation for a time, then stop working. In paints and stains, the pigments block UV the same way a long-sleeve shirt protects exposed skin from sunburn. UV breaks down lignin, the natural glue that holds wood fibers together, and eventually turns all wood left outdoors as gray and rough as barnboard. Most of these coatings rely on oils-natural linseed and tung or synthetic resins-to resist moisture on preservatives containing zinc, iodine, borates, and other compounds to discourage mildew, moss, and mold from taking hold and on a combination of ultraviolet-absorbing and blocking ingredients for the toughest task of all-blocking UV degradation. That's because the three main agents of wood decay haven't changed. The result has been a host of effective, durable new coatings for decks, siding, fences, and log homes.įor all the sophisticated chemistry, the basic ingredients of every exterior finish remain remarkably similar. "A lot of companies, particularly in Europe, took the Madison idea and built on it," says Johannes Boonstra, technical support specialist for Sikkens Wood Finishes. This blend of linseed oil, earth pigments, mildewcides, and paraffin, called the Madison formula, inspired coatings technologists around the world. Many of these improvements can be traced back to the 1950s when researchers at the federal government's Forest Products Laboratory, in Madison, Wisconsin, created a new category of finish: the penetrating sealer, which offered more waterproofing than any penetrating finish before it. In recent years the distinctions between the two types of finish have begun to blur as manufacturers develop formulas that offer the protection and gloss of a film with the maintenance ease of a sealer. Photo by David Carmack Improvements On the Formula "When a clear finish is showing off the beauty of mahogony, white oak, walnut, or antique pine, it's definitely worth the effort," says painter John Dee, who applied three coats to the entry doors at This Old House's Charlestown project. But even the best ones need a routine reapplication just as often as film-formers, and do little to guard the wood surface from dirt and wear. Compared with hard coatings, they do a better job of letting damp wood dry out, and they can be recoated without elaborate surface preparation. Penetrators, on the other hand, preserve wood by soaking into its fiber and so do not peel or require scraping or sanding the finish simply wears away. But they're often demanding to apply and always unforgiving of neglect: If not lightly sanded and recoated every one to three years, the film will begin cracking and peeling, and then must be stripped down to bare wood. The film-forming products-both classic varnishes and modern urethanes-are unmatched in their ability to bring out the beauty and depth of a wood surface while guarding against wear and tear. Assaulted by water, ultraviolet light, and mildew spores while they try to cope with wood's tendency to shrink and swell, some finishes peel like sunburned skin or are little more than solvents that just evaporate into thin air.įortunately, advances in coatings technology have created new formulations that actually live up to their billing, as long as they are lovingly maintained.Ĭlear finishes work in one of two ways: either by forming a hard film over wood or by penetrating it. The truth is, though, that shielding wood from the elements without resorting to an opaque blanket of paint is not that easy. ![]() Judging by the labels on the cans of exterior-grade clear finishes at paint stores and home centers, the job of keeping wood looking good seems downright easy: Just brush one of these products on a mahogany entryway, a redwood deck, or cedar siding, and the wood will stay as fresh and bright as the day it was cut. ![]() "Guaranteed to prevent graying." "Restores wood's moisture content and helps it stay flexible." "Stops cupping, cracking, curling, and warping." Photo by David Carmack How Clear-Coats Work ![]()
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