Should I pressure wash my deck, or use a cleaner and brightener?

What pressure washing really does to wood

Softwood decking, which is most redwood, cedar, Douglas fir and pressure-treated lumber, has alternating bands of soft springwood and harder summerwood in every board. A pressure washer tears the soft bands out faster than the hard ones. The result is a washboard texture, fuzzy raised grain, and wand marks that look like someone mowed the deck.

Those marks are permanent. Sanding can reduce them, but a deck that has been aggressively pressure washed will never finish as evenly as one that was cleaned chemically. We see this on a lot of decks in North County, usually from a rental washer and a good intention.

There is a place for a pressure washer in deck work: rinsing. At low pressure, with a wide fan tip, kept moving and held well off the surface, it rinses cleaner and brightener faster than a hose. That is the only way we use one on decking.

How a percarbonate cleaner works

Sodium percarbonate is oxygen bleach. Mixed with water it releases hydrogen peroxide and soda ash. The peroxide oxidizes the gray, UV-damaged fibers and kills mildew, and the alkaline soda ash loosens dirt and old oil finish residue. It does its work in ten or fifteen minutes of dwell time while the surface is kept wet, then a stiff brush and a rinse carry the gray away.

The important difference from chlorine bleach is that percarbonate does not attack the lignin that holds sound wood together. Household bleach whitens wood by destroying it a little. Percarbonate cleans it without that damage, and it is far easier on plants and on the people using it.

Percarbonate will also pull loose, chalky remnants of an old oil finish. It will not remove a film finish such as varnish or acrylic stain. Those need a dedicated stripper.

Why the brightener is not optional

After the cleaner, the wood is clean but it is also alkaline and often slightly darkened. An oxalic acid brightener does three things at once. It neutralizes the alkaline cleaner so the finish will cure properly. It dissolves iron stains, the black streaks around nails and screws. And it lightens tannin discoloration so redwood and cedar come back to their warm natural color.

People who skip the brightener tend to get a deck that looks a bit dull and gray-brown after finishing, and sometimes a finish that stays tacky. Those are both symptoms of finishing over an alkaline surface.

Oxalic acid is a real acid. Gloves and eye protection are not optional, and it needs a thorough rinse so none of it is left in the wood.

The sequence we follow

This is the order that produces a clean, open, evenly colored surface ready for oil.

• Sweep the deck and clear the gaps between boards with a putty knife so water drains and cleaner reaches the edges.
• Wet down nearby plants and the siding, and keep them wet while the chemicals are on the deck.
• Apply the percarbonate cleaner with a pump sprayer or a mop, working in sections small enough to keep wet. Let it dwell, then scrub with a stiff synthetic brush on a pole.
• Rinse thoroughly with a hose or a washer on low pressure and a wide tip, working with the grain.
• Apply the oxalic brightener the same way. The color change is usually visible within minutes. Rinse again, very thoroughly.
• Let the deck dry for two or three warm days before any finish goes on. Finishing wet wood is the most common cause of early failure.

Mildew, algae and the marine layer

Shaded decks near the coast grow mildew and algae because the marine layer keeps them damp into the day for a good part of the year. Percarbonate handles that during a cleaning. Between cleanings, improving air flow under the deck, trimming back plants, and keeping the board gaps clear does more than any chemical to keep it from coming back. Finishes with a mildewcide help, but they are a supplement, not a solution.

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