Why does my teak furniture get black spots near the coast, and how do I fix them?

Two kinds of black spot

Most dark spotting on outdoor teak is mildew, a surface mold that grows on dirt, pollen, sunscreen and especially on oil finishes. It looks like scattered gray-black freckles, often heaviest on the undersides of arms, in joints and on shaded faces. It wipes partly off with a thumb and comes right back.

The other kind is iron staining. Teak is high in tannins, and tannins react with iron to make a blue-black stain that does not wipe off at all. Common sources are steel wool used in cleaning, rust from a nearby fastener or a steel table base, and water that has run off a rusty bracket. A test tells them apart: oxygen bleach lightens mildew but not iron; oxalic acid lifts iron but does little for mildew.

Why the coast makes it worse

Carlsbad and the rest of the North County coast spend a large part of spring and early summer under a marine layer that rolls in overnight and burns off late in the morning. Furniture gets wet every night and stays damp for hours every day. That is close to ideal for mildew.

Salt in the air adds to it. Salt is hygroscopic, which means it pulls moisture out of the air and holds it on the surface. A piece of teak with a season of salt film on it stays damp longer than a clean one. That is why a plain rinse with the hose now and then does more for coastal teak than most people expect.

Shade is the third factor. Teak under a patio cover or tucked against a north wall dries slowly, and mildew follows the slow-drying spots.

How we clean it off

The process is the same one we use to restore gray teak, because mildew and graying arrive together.

• Wet the piece down with a hose.
• Apply a sodium percarbonate cleaner, an oxygen bleach, mixed per the label. Keep it wet and let it dwell for ten or fifteen minutes.
• Scrub with a soft brush along the grain, getting into joints and undersides where the mildew lives.
• Rinse thoroughly.
• If dark spots remain, apply an oxalic acid brightener, let it work for a few minutes, and rinse again. This also evens the color.
• Dry for several days in the shade before any sealer goes on. Never seal or oil over mildew; it will grow back under the finish where you cannot reach it.

Keeping it from coming back

You cannot change the marine layer, but you can change what the mildew has to eat and how long the wood stays wet.

The biggest single change is to stop using teak oil. Linseed and tung oils are food for mildew, and most badly spotted teak we see has an oil history. If you want the honey color, switch to a dedicated teak sealer with a mildew inhibitor, applied only to clean, dry wood. If you do not need the color, leave the teak bare; mildew still grows on bare teak but far less, and it rinses off.

Beyond that, give the furniture room to breathe. Pull it a foot away from walls, take cushions off when they are not in use so the seat slats dry, and rinse the salt film off with the hose every few weeks in the damp season. If a piece lives under a cover, open the cover or move the piece into the sun for a day now and then.

When the spots are not on the surface

Occasionally a black mark goes deeper than cleaning will reach: a nail or screw that has rusted inside the wood and stained a ring around itself, or a check in the grain that has held water and mildew for years. Those get a light sanding after cleaning, and if a fastener is the cause, we replace it with stainless. Solid teak is thick enough to lose a little surface and still have plenty left.

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