STAND UP TO THE SUN
Finish south-facing wood with paint for best protection from elements
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 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, use 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 fibres. The fibres 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 being 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.
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.
Water-based semitransparent and solid-body stains in a full range of colours 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.
What do you do now? Begin by washing the surface. Then get off as much of the loose finish and grey wood fibres 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 labelled for exterior use over bare wood. If it seems impossible or infeasible to remove all of the grey wood, Frank Glowacki, brand director for Rust-oleum, which makes a 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 grey wood fibres. 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 cannot 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 mid afternoon is generally safest.