Deck Staining & Sealing on San Juan Island

If your deck has soft boards, a railing that moves, or a sagging stair, that is deck and fence repair and it comes first. This page is the other half of the job: the coating. What goes on the wood, how the wood gets ready for it, and when in the year it can actually happen.

Start with why island decks fail unevenly. Walk any waterfront deck here and the south and west elevations — the boards nearest the open water, the top rail, the stair treads, the west-facing fascia — will be gray and open-grained while the sheltered corner by the house still has finish on it. That is ultraviolet and reflected light off the water, not rain. Meanwhile the north side of the same deck is holding algae and moss in the board seams, because it never fully dries between October and April. Same deck, two different failures, and a single coat of anything applied over both without addressing either will not last.

Prep is three separate steps and skipping any of them is the usual reason a finish fails early. First, clean: a percarbonate or sodium hydroxide deck cleaner to lift the biological growth and the gray dead wood fiber, agitated and rinsed, at low pressure. Pressure washing a cedar or fir deck at high pressure furs the grain and the stain then soaks in unevenly and blotches. Second, brighten: an oxalic acid brightener to neutralize the cleaner and bring the pH back down, which also pulls the black tannin staining out around the fasteners. Third, dry. Genuinely dry, all the way through the board, not dry on top.

That third step is where the island calendar bites. Friday Harbor is in the Olympic rain shadow and gets a real dry summer, but the useful window is narrow: roughly late May through mid-September, with July and August the only months you can plan on. A deck needs two to three consecutive dry days after washing before it takes a finish, plus dry weather after application. Boards that are shaded, tight-spaced, or over a damp crawlspace hold moisture much longer than the surface suggests, and I check with a moisture meter rather than a thumb. If it is not ready I will tell you and we move it, because staining damp wood is the single fastest way to waste the whole job.

On product I have a clear opinion for this climate. Penetrating oil finishes over film-forming ones, nearly every time. A film-forming stain or a deck paint sits on top of the wood, and in a climate where the boards take on moisture every winter, that film eventually lifts, cracks, and peels — and then you are into stripping or sanding, which is the expensive way to own a deck. A penetrating oil soaks into the fiber, wears off gradually rather than failing in sheets, and recoats by cleaning and reapplying. It needs doing more often, on a horizontal island deck realistically every two to three years on the sun-hit elevations and less on the sheltered ones, but each cycle is cheap and there is no failure mode where it has to come off.

If your deck already has a film-forming coating on it, say so before I quote, because that changes the job entirely. Recoating over a peeling film is not a repair, and stripping or sanding a full deck is its own project. On a deck that already has a solid stain on it and is otherwise sound, the honest recommendation is often to stay on the same system rather than fight it.

Railings and stairs get finished properly, not sprayed at from a distance. The top rail and the stair treads take the most water and the most wear and they are the parts anyone actually touches, so they get worked in by hand. Fasteners get checked while I am down on the boards — a corroded screw head on a waterfront deck is worth catching, and it is the sort of thing you only see when you are on your knees with a brush.