Finishing a deck in high desert sun

Wood decks in the Treasure Valley go silver-gray faster than most people expect, and the reason is specific. Ultraviolet light breaks down lignin, the compound binding wood fibers together. As the surface layer degrades it loses color and cohesion, and what is left is loose gray fiber. At around 2,600 feet with a long clear summer, this valley delivers a heavy annual ultraviolet dose.

The gray itself is cosmetic. What comes with it is not. Surface checking, the fine splits that open along the board face, lets water into the wood, and water is the mechanism behind rot. Finishing a deck is really about slowing the light, and the water problem takes care of itself.

The first coat on a new deck

A brand new pressure treated deck usually should not be finished immediately. Standard wet treated lumber arrives saturated from the treatment process, and a penetrating stain cannot get into wood that is already full of water. It sits on the surface, fails to bond, and peels within a season.

The test is the same water sprinkle. When the boards absorb water rather than beading it, they are ready. In a Meridian summer that can be a few weeks. Following a wet spring it can be considerably longer. Kiln dried after treatment lumber is the exception and can generally be finished much sooner, which is one of several reasons it is worth the upcharge.

Western red cedar is different again. It arrives dry and stable and can usually take a finish once the surface mill glaze is dealt with. Cedar rewards being finished early, because there is more worth protecting.

What to put on it

Three broad categories, and in this climate they do not perform equally.

• Clear water repellent sealer. Blocks water, blocks almost no ultraviolet light. Looks excellent for one season, then the wood greys underneath it. Reasonable on a covered deck, weak in open sun.
• Semi-transparent stain. Pigment plus penetrating oil or resin. The pigment is what does the ultraviolet work while the grain still shows. This is the workhorse for exposed decks here, and typically gets two to four years.
• Solid color stain. Effectively an opaque coating. Longest ultraviolet protection, hides the grain entirely, and it can peel rather than wear because it sits more on the surface. Once a deck goes solid it is difficult to go back.

A useful rule: the more pigment a product carries, the longer it lasts and the less it looks like wood. Where a deck lives in full afternoon sun, lean toward more pigment than you think you want.

The re-coat interval

Two to four years for most Treasure Valley decks. The spread is almost entirely about exposure. South and west facing surfaces with no shade sit at the short end. North facing decks and anything under a cover stretch to four or five years. Horizontal surfaces need it long before vertical ones, so a rail can still look fine while the deck floor has given up.

Do not run this on a calendar. Run the water test each spring on the sunniest section of the deck, and act when it stops beading rather than when the calendar says so.

Bringing a gray deck back

Weathered wood usually cleans up better than people expect. The sequence that works:

1. Clean. A wood deck cleaner, applied and given its dwell time, then agitated with a stiff brush and rinsed. This lifts dirt, mildew, and the degraded surface fiber.
2. Brighten. An oxalic acid based brightener neutralizes the cleaner and restores tone to wood that has gone dark or blotchy. This step is skipped constantly and it is the one that decides how the finish looks.
3. Dry. Twenty-four to forty-eight hours in a Meridian summer, longer in spring. Coating over damp boards is the single most common cause of an early failure, whatever the product.
4. Sand where needed. Boards with raised grain or deep checking need sanding rather than washing. Badly cupped boards need replacing.
5. Coat. Half a day on a typical deck, working with the boards rather than across them, and keeping a wet edge.

Total: two to four working days on a 400 square foot deck, most of it waiting for the boards to dry. Any schedule that puts the wash and the coat on the same day is worth a question.

What not to do

• Pressure wash on a narrow tip. It erodes the soft early wood between growth rings, leaves a corduroy texture, and drives water into checks. If a pressure washer is used at all, it should be a wide fan at low pressure, held well back.
• Coat in full sun. The finish flashes off before it can penetrate. Work in shade or in the cooler part of the day, which in a Meridian July means early morning.
• Coat over a failing film. Solid stains and film-forming products have to be stripped rather than recoated once they start peeling.
• Assume the rail is on the same schedule. Vertical surfaces shed water and take less direct sun, so they routinely last twice as long as the deck floor.

The honest version of the trade

Over twenty years, a wood deck in this valley will want six or seven refinishing cycles. At $1,400 to $3,200 per cycle on a 400 square foot deck, that is real money and real weekends. That number is exactly what the composite argument is made of, and it is why the materials comparison comes out differently here than it would in a milder, shadier climate.

None of which means wood is the wrong choice. Cedar underfoot in August beats any synthetic board, and plenty of people happily trade a weekend every third spring for that. It just should be a decision made with the number in view.