Why are there black streaks around the nails on my deck and fence, and how do I remove them?

What is actually happening in the wood

Redwood, cedar, oak and many tropical woods are full of tannins, the same compounds that make strong tea dark. Tannins are part of why these woods resist rot and insects. They also react with iron. When a fastener starts to corrode and rain or fog carries a few dissolved iron ions into the surrounding grain, the iron and tannin combine into a black compound that is essentially the same chemistry as old-fashioned ink.

The stain spreads in the direction water travels, so you usually see a dark halo around the fastener with a tail running downhill. On a fence the tails run down the pickets. On a deck they pool at board ends and low spots.

Iron stain versus tannin bleed

Homeowners often lump two different problems together. Iron stain is black or blue-black, centered on metal, and does not wash away with soap. Tannin bleed is a brown or reddish wash that shows up as streaks under the wood after rain, or as brown blotches through light-colored paint. Tannin bleed is the wood's own extractives migrating to the surface and it is worst on fresh redwood and cedar in the first year or two.

The two are treated differently. Iron stain is removed with oxalic acid. Tannin bleed on raw wood is also lightened with oxalic acid, but when it is coming through paint the answer is a stain-blocking primer before repainting, because no amount of washing stops it from the top side.

How to remove the stain

Oxalic acid, sold as wood brightener or deck brightener, is the right tool. It is the same step we use after cleaning any gray deck, so if the whole surface is being restored the stains disappear as part of the process.

For a spot treatment on an otherwise finished surface, the stain has to be bare to the wood. A film finish over the stain will block the acid, so the spot is sanded first and then refinished afterward.

• Wet the wood first so the acid does not soak in unevenly and leave a light patch.
• Mix the brightener per the label, apply with a synthetic brush or pump sprayer, and keep it wet for several minutes. The black usually lifts while you watch.
• Rinse thoroughly. Oxalic acid left in the wood will interfere with the finish and can leave a white bloom.
• Wear gloves and eye protection and keep it off plants and concrete, which it can etch.
• Let the wood dry fully before any finish goes on, which near the coast typically means a couple of clear days.

Stopping it from coming back

Removing the stain treats the symptom. If the fastener keeps corroding, the halo returns within a season. On any deck or fence near the ocean we use stainless steel fasteners on exposed surfaces, especially with redwood and cedar. Hot-dipped galvanized is the next best choice and is acceptable on framing that stays dry, but electroplated or bright steel fasteners have no place outdoors here.

The other common iron source is us, not the hardware. Steel wool, a steel wire brush, or a sander that was last used on metal will leave fine particles in the grain that turn black with the first dew. We keep brass or synthetic brushes for tannin-rich wood and wipe tools before they touch it.

When a black spot is not an iron stain

A black area that is soft when you press a screwdriver into it, or that is spongy around the fastener rather than crisp, is rot, and rot around a fastener often means water is getting in along the shank. Mildew also reads as black specks but sits on the surface and wipes off with a cleaner. Iron stain is hard, sharp-edged and ignores soap. If you are unsure, probe it. Sound wood under a black halo is a cosmetic problem; soft wood is a repair.

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