Choosing an Exterior Coating

The need to protect wood outdoors is much greater than the need to protect it indoors because of exposure to sunlight and rain. These cause wood to gray, split, warp and rot; and moist conditions make the growth of mildew possible.

You can use paint, stain, clear finish, water repellent and preservative to prevent or retard damage to exterior wood. But first, it’s helpful to understand the causes of the damage.

Exterior Damage
Sunlight contains strong ultraviolet light, which is very destructive to wood over time. UV light destroys the lignin that glues the cellulose wood cells together, and rain then washes the lignin away. Because the lignin contains the extractives that give wood its distinctive coloring, the wood turns silvery gray on the surface when the lignin is gone.

Sunlight also heats the surface of the wood and draws out moisture, causing shrinkage. This leads to splitting and warping, and these are made worse by rain when it comes in contact with only one side of the wood – as on decks, tabletops and exterior doors. The water makes the surface cells swell, but the thickness of the wood prevents the surface from expanding. The cells are then forced to compress to oval shapes, and they hold these shapes even when dry.

This phenomenon is called “compression shrinkage” or “compression set,” and I described it in the context of finishing both sides of wood in the October 2004 issue. (Back issues are available online at popwood.com.) Compression shrinkage causes wood to warp and split as the exposed side continues to shrink a little more each time it goes through the wetting and drying cycle.

Rain is partially responsible for rotting and the growth of mildew, because both require moisture to occur. Rain is also indirectly responsible for a visually similar damage – insect infestation – because insects require moisture to thrive.

The heartwood of redwood, cedar and some hardwoods is naturally resistant to rotting. Some softwoods are pressure treated with chemicals to make them resistant to rotting. These woods have the familiar dull green or dull brown coloring. Sapwood and non-pressure-treated pine and fir are not resistant to rotting.

There are five different types of coatings you can use to protect against the problems caused by sunlight and rain: paint, stain, clear finish, water repellent and preservative. You can buy any of the first four types of coatings with a preservative included to retard mildew, or you can sometimes buy a concentrated preservative separately and then add it yourself.

Paint
Paint is the most effective coating for protecting wood. The thick film blocks water penetration and the pigment blocks UV light. You can find wood siding that is in perfect shape after 200 years because it has been protected continuously with well-maintained coats of paint.
There are two large categories of paint: oil-based and water-based (latex). Because oil-based paint wears better than latex paint, it is best for objects that see a lot of abuse such as chairs and picnic tables.

Oil-based primers are also best when you are painting wood that has been exposed to the weather for a month or longer, especially if the wood has grayed. Oil-based primers penetrate deeper than latex primers, so they are better able to penetrate the degraded wood caused by the destruction of the surface lignin and bond to good wood underneath. If the wood is freshly milled or sanded, acrylic-latex primers perform well.

Latex paint is best for wood siding because it is better than oil paint at allowing moisture vapor created inside a building to pass through. If the moisture vapor can’t get through the paint layer, it builds up behind the paint and causes it to peel. (A primer coat of oil-based paint applied under latex paint is not thick enough to stop moisture penetration.)

Paint is great for siding and house trim because they can be caulked to keep water from getting into the wood and causing the paint to peel. Paint is also great for furniture and exterior doors if they don’t get a lot of exposure to moisture.

But paint is a poor choice for decks and often for fences because it’s rarely possible to seal off all the end grain effectively. The paint peels and requires too much work to effectively keep up.

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