How Clear-Coats Work
“Guaranteed to prevent graying.” “Restores wood’s moisture content and helps it stay flexible.” “Stops cupping, cracking, curling, and warping.”
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.
The truth is, though, that shielding wood from the elements without resorting to an opaque blanket of paint is not that easy. 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.
Fortunately, advances in coatings technology have created new formulations that actually live up to their billing, as long as they are lovingly maintained.
Clear finishes work in one of two ways: either by forming a hard film over wood or by penetrating it. 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. 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. 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. Compared with hard coatings, they do a better job of letting damp wood dry out, and they can be recoated without elaborate surface preparation. 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.
Improvements On the Formula
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. 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. This blend of linseed oil, earth pigments, mildewcides, and paraffin, called the Madison formula, inspired coatings technologists around the world. “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. The result has been a host of effective, durable new coatings for decks, siding, fences, and log homes.
For all the sophisticated chemistry, the basic ingredients of every exterior finish remain remarkably similar. That’s because the three main agents of wood decay haven’t changed. 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. 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. In paints and stains, the pigments block UV the same way a long-sleeve shirt protects exposed skin from sunburn. Clear coatings behave more like sunscreen: They contain chemicals that absorb UV radiation for a time, then stop working. “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. Once the chemicals lose their ability to absorb UV—in as little as a few months in sunny climates—the rays pass through unhindered and start to deteriorate the wood surface. Then it’s only a matter of time before the wood starts fraying, causing the finish to lose its grip.
About UV Resistance
Adding pigment is the surest way to block UV over the long run. If it closely matches the natural color of the wood and is used in limited quantity, it isn’t noticeable. Add too much, however, and the coating begins to darken the grain and resemble a stain. Still, a new class of pigments, first developed for automobile paint in the 1970s, provide UV resistance without compromising clarity. Called transoxides, or transparent iron oxides, they are particles ground so finely that they literally fit between the wavelengths of visible light. In effect, the rays we can see pass through virtually unimpeded, while most of the shorter UV waves reflect back and scatter before they reach the wood.
Knowing how finishes work is all well and good. The problem is, manufacturers jealously cloak their ingredients in secrecy, so there’s no way for consumers to know or compare how much or which kind of pigments, UV absorbers, or preservatives there are in the can. To make matters worse, labeling requirements are maddeningly lax. For instance, the word “preservative” on a label means only that the government approved the relative safety of the fungicide and accepted evidence that it killed some organisms in whatever concentration it was tested. Manufacturers don’t have to use this same concentration in their formulas, nor do they need to meet any standard for overall effectiveness.
About all a consumer can do is roughly infer a finish’s quality. You can do this by getting hold of the Manufacturers Safety Data Sheet (MSDS), available from the retailer, and subtracting the solvent content (which has to be listed) from the total product amount. That will give you an approximate idea of the amount of solids—the ingredients that do the work—that will be left after the solvents evaporate. Solvents are important for delivering good penetration or a smooth film, but the product should be more than just evaporative liquids. Another way to judge quality is to look at the price tag: A cheap finish won’t have a lot of the expensive ingredients that ensure durability.
Still, that leaves a lot of unanswered questions about which product to buy. Sam Satterwhite, a log-home builder in Longview, Texas, became so frustrated and confused that in the mid-1980s his company teamed up with Texas A&M University to test clear and lightly pigmented finishes on a series of log walls, milled siding, and rough plywood. They found that TWP (a penetrator) and Cetol (a two-layer product with a penetrating sealer followed by a hard coat) came out best of 13 sealers tested. Satterwhite favors Cetol because the film helps bridge the cracks that develop in wood as it dries. Peeling can occur if the required maintenance coats aren’t applied according to manufacturer’s suggestions. “But the finish doesn’t come off in sheets like some products do,” he says. “If it’s caught early on when there are just little cracks, a minor amount of sanding will remove the loosened material and prep the wood surface for reapplication.” Cetol, however, should be applied by brush. If his customers want a coating they can maintain by spraying, Satterwhite recommends TWP.
John Dee, a Massachusetts painter who has worked with This Old House, has never used TWP, but he’s seen Cetol in action on decks. Although many paint experts say using film-forming finishes doesn’t make any sense on these heavy-wear horizontal surfaces, Dee’s not so sure anymore. “I’ve watched two decks with the film coating — one that wasn’t maintained, and it peeled; and one that was maintained, and it looks great,” he says. “That maintenance coat actually sort of melts whatever’s underneath so that you don’t get heavy buildup year after year.”
In the end, the decision of whether a clear coating is worth the trouble is one of aesthetics versus effort, says Dee. “You need to really like the look of natural wood to be willing to do the maintenance.”
Sparring Partner: What to Use Outdoors
When you want the warm, welcoming look of a nicely varnished front door, nothing beats the real thing. Marine spar varnish is the best type for outdoor use. Its key ingredients are linseed oil (pressed from flax seeds) and alkyd resin (made by reacting linseed oil with alcohol and acid). The film is flexible enough to move with wood as it shrinks and swells with shifts in humidity. Mark Knaebe, of the Forest Products Laboratory, recommends selecting a spar varnish with UV absorbers and applying up to six thin coats for maximum protection. To keep the wood looking good, sand lightly and brush on a fresh coat every year. Otherwise, the varnish will become brittle and crack. And once that happens, you’ll have no choice but to scrape it all off and start over. — J.H.
Despite the dozens of clear-finish choices on home-center shelves, there are basically two types—those that soak into the wood fibers like stain and those that form a shell on top like paint.
1. Urethane: Forms a hard film; choose an acrylic product.
2. Penetrating sealer: High solids content improves durability.
3. Hybrid: The first coat soaks in, the second provides a protective film.
4. Varnish: This ancient film-forming product requires 6 to 10 coats.
5. Penetrating oil: Nut extracts soak deeply into dense wood.