Grey decks, docks and fences: what weathered wood can come back from

Sunlight breaks down lignin, the natural glue that holds the fibers at the surface of the wood together. Rain washes the broken-down lignin away and leaves loose, bleached fibers behind, and those look silver-grey. That is weathering, and it is the wood version of the oxidation that chalks siding.

Mildew and algae then move into the loosened surface, which is why weathered wood goes from silver to a blotchy dark grey or black in shaded and damp areas.

More than most people expect. Much of what reads as grey on a deck is actually dirt and mildew sitting in the grain, and cleaning that out is the big visible change.

What cleaning can't do is replace wood fiber that the sun has already destroyed. A deck that has been bare for a decade will come back clean, even and much lighter — but it will look like weathered wood, not like new lumber. Bringing back more of the original tone is done with a wood brightener, which is a separate step from cleaning.

Weathered wood is soft on top. Hit it with too much pressure and the wand tears the loose fibers up rather than cleaning them, leaving a fuzzy, splintery surface — called furring — and sometimes visible lines gouged along the grain. It is permanent short of sanding.

That is why decks and fences are cleaned at a pressure the boards can take, working with the grain, and why a deck that was blasted once by someone else often shows it for years.

Composite doesn't go grey the way wood does, but it has its own version. Older composite boards, the kind without a hard plastic cap, grow mold and mildew in the wood fibers inside them and fade noticeably in the sun. Cleaning removes the growth. The fading, like oxidized siding, stays.

Up north, the wood that sees the hardest life is on the water — docks, boathouses and lakeside decks, wet from below and in full sun from above. Cedar siding and cedar fencing weather the same way and grow the same mildew, and they need the same light touch.

Cleaning is the step before stain, not an optional extra. Stain applied over mildew and loose grey fiber doesn't bond to the wood and peels early, taking the money spent on it with it.

After cleaning, the wood needs to dry out properly before anything goes on it — which means a spell of dry weather, not an afternoon. Whoever is doing the staining will have a view on how long; the product label will too. Plan the two jobs with that gap in between.