Sunlight and water can ravage wood. Ultraviolet light can bleach it gray, and water can cause it to warp, split and rot. That’s why finishing exterior wood surfaces, which must withstand exposure to these destructive elements, requires a different approach from that used for interior woodwork and cabinets.
Here’s how you can use the three types of coatings -- paint, stain or clear finish -- to protect exterior projects.
Paint
Paint is by far the most protective finish because it can be applied to a thickness that prevents water penetration and because it contains pigment that blocks UV light. There are two broad categories of paint: oil-base, which requires mineral spirits (paint thinner) for thinning and cleanup, and water-base (latex), which can be thinned and cleaned up with water.
Because of environmental concerns, oil-base paint is becoming difficult to get in some parts of the country. It has a strong odor, dries slowly and is difficult to clean, but it bonds better to grayed, dirty or chalky surfaces than water-base paint does.
Water-base paint is more user-friendly, has very little odor, dries fairly quickly and cleans up easily with soap and water. (Though many people consider fast drying an advantage, it can be a problem if you’re painting in sunlight. You can slow the curing by adding a product called “Floetrol.” It’s sold at most paint stores.)
Both types of paint are available in primer form, which contains a higher ratio of binder (the part that bonds to the surface) and a lower ratio of pigment. Primers also may contain additives to help resist mildew or rust. You should apply a primer coat whenever you’re painting bare wood, grayed wood or old paint that has begun to chalk or crack.
Stain
Exterior stains are also available in oil and water-base formulas. As with paint, oil-base stains bond better but are more difficult to clean up, slower to dry and more odorous than water-base.
The key difference between exterior- and interior-rated stain is the type of pigment used. Exterior stains contain pigments that resist UV light better than the pigment in interior stains, which are designed to be more transparent. In fact, you can think of an exterior stain as thinned paint.
The main advantage of exterior stains over paint and clear film-building finishes is that they don’t peel because they don’t build up on the wood. That’s why stain is better than paint for decks, fences and some types of siding (those that can’t be caulked to prevent water from getting underneath and causing peeling).
Stain offers some resistance to water penetration because of the binder and some resistance to UV light because of the pigment. And because it doesn’t build up on the wood, there’s never a need to scrape or sand, so you can apply additional coats with relatively little preparation — simply clean off the dirt by pressure washing or scrubbing with a stiff broom and hose. The downside is that you have to recoat much more often: once every year or two, or whenever the color begins to wear through.
Clear finish
There are three types of clear finish for exterior wood: water repellant, oil, and film-building varnish or clear water-based finish. None of them protect very well.
Water repellant provides almost no resistance to UV light, so it allows wood to gray very quickly. And because there’s no binder, just a noncuring oil or wax that causes beading for a short time, the resistance to water penetration is minimal. Ease of application and misleading advertising have lured many people to try this type coating, often with disappointing results.
Oil finishes, including so-called teak oils, offer only a little more protection than water repellant. Water and UV light break down oil finishes quickly.
Film-building varnishes and water-base clear finishes provide excellent water resistance if you apply several coats, but only marine varnishes offer any significant UV resistance. These varnishes (and some water-base finishes) contain a significant amount of UV absorbers, which break down UV light. Applying many coats of finish creates a thick layer of absorbers, which is fairly effective at blocking UV rays for a few years.
The problem is that the top layers of finish break down in sunlight and have to be sanded off and replaced with additional coats every year or two, depending on the amount of sun exposure and the latitude (how far south you live). Marine varnishes also cost considerably more than other varnishes. (Be warned that the “marine” varnishes sold in home centers do not have anywhere near the same UV resistance as the ones sold at marinas for use on boats.)
Marine varnishes are a reasonable option for exterior doors because sunlight strikes them at an angle and for only part of the day. But the best solution is an overhang that protects against both UV light and rain, which can get through joints and cracks and cause peeling.
Bottom line
The bottom line is that paint always provides the most protection against UV light and rain, but on surfaces such as decks that can’t be totally sealed off from water penetration, a stain is best because it doesn’t peel. And except for marine varnishes, clear coatings have very limited effectiveness against sunlight.
Bob Flexner is author of Understanding Wood Finishing, Fox Chapel Publishing, 2005.