South-facing wood should be finished with paint, not varnish
By Jeanne Huber,
Q: The south-facing side of our screen porch gets intense sun. Two years ago, it was sanded, stained and varnished with what we thought was the best exterior marine varnish. Now it needs to be completely redone. Needless to say, this is expensive and frustrating. Is there a better solution than refinishing the window frames every couple of years?
A: Especially on south-facing wood, go with paint, not marine varnish or stain topped by marine varnish.
When ultraviolet rays from the sun hit wood, they break down lignin, the natural glue that bonds wood fibers. The fibers then flake off, carrying the finish with them.
Because paint is opaque, it blocks light waves from reaching the wood. So the finish stays on, assuming the surface was prepared properly before it was painted. A well-done paint job should last for many years, even on the south side of a house.
Marine varnish is a clear finish that contains UV absorbers. These photo-reactive ingredients deactivate UV rays, but they are sacrificial: Once used, they no longer work. That’s why people who depend on marine varnish for the “bright wood” look of natural wood on boats apply many layers. The more varnish on the surface, the longer the wood will stay protected. But as anyone who’s owned a wooden boat with natural-looking wood knows, marine varnish doesn’t last as long as people wish. When the UV protection wears out, the finish needs to be stripped and redone.
Stains can also protect wood from UV damage, but their effectiveness varies. When someone wants to keep the look of natural wood, the best choice is a type that contains transparent iron oxide pigments — particles ground to a size that physically blocks UV wavelengths but allows much of the visible part of the light spectrum to scoot through. Unlike the UV absorbers built into marine varnish, these UV blockers offer permanent protection — as long as the finish doesn’t erode or flake off. But the lighter the color, the less protection. Also, because iron oxide comes in red, yellow and brown, these stains are available in wood tones only.
Water-based semitransparent and solid-body stains in a full range of colors are also available and helpful in blocking UV. Solid-body stains, which are nearly as opaque as paint, offer more UV protection than thinner stains. (From the picture you sent, it appears you used a beige-colored solid-body stain.) On a deck or other outdoor, horizontal surface, the big selling point of these stains is that they are less likely to peel than paint, which is likely to fail. But multiple coats of solid-body stain can still peel, and adding varnish on top of the stain makes this even more possible. Layering different types of products that weren’t formulated to go together introduces the risk that the two types will expand and contract at different rates as temperature and humidity shift. Marine varnish comes in both water- and oil-based formulas, so without knowing the specific products you used, it’s impossible to say whether this would explain the peeling you’re seeing. There are exceptions, but in general, it’s okay to top an oil-based finish with a water-based one, because the water-based finish stays more flexible over time and can accommodate movement in the oil-based finish underneath. But topping a water-based finish, including a solid-body stain, with an oil-based finish can lead to peeling. There are other possible explanations for your problem, including temperature and moisture levels at the time of the painting.
What do you do now? Begin by washing the surface. Then get off as much of the loose finish and gray wood fibers as possible. Because you redid the finish two years ago, you don’t need to worry about lead dust, so sanding may be the quickest solution.
If you get down to bright wood, use a water-based primer labeled for exterior use over bare wood. If it seems impossible or infeasible to remove all of the gray wood, Frank Glowacki, brand director for Rust-Oleum, which makes a wide variety of paints and primers, recommends priming with Zinsser’s Peel Stop Triple Thick. This primer and the less-thick original Peel Stop formula stick far better than regular primer when surfaces have remnants of peeling finishes or gray wood fibers. Peel Stop Triple Thick is $30.98 a gallon at Home Depot. Top whatever primer you use with two coats of exterior water-based paint.
Read labels and follow instructions about the range of suitable air and surface temperatures. For water-based finishes to cure properly, the water can’t evaporate too slowly or too quickly. A day that seems moderate can quickly get quite hot when the sun shifts or too cold when the sun sets. For a south-facing wall, painting in midafternoon is generally safest.
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