Design & DIY

Cottage Q&A: Cleaning black stains off cedar

Someone pressure-washing a stained fence By Roschetzky Photography/Shutterstock

I have a cedar fence at the cottage. Over the years, it’s gotten these black stains on it. What causes this? How do I remove them? —Hamish Connor, Midland, Ont.

A number of sources could be responsible for the stains, says Rod Stirling of FPInnovations in Pointe-Claire, Que. “I can’t be sure without seeing them.” 

If the black marks are around the nail heads, it could be an iron stain, says Stirling. “Iron fasteners can react with natural chemicals in wood to form black compounds.” You can remove the stains with oxalic acid—look for deck cleaners that contain it. Keep in mind that oxalic acid is corrosive and harmful to plants, so be careful when you’re applying it.

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If the stains are widespread, one likely culprit could be black stain fungi, says Stirling. Good news: “These are non-decay fungi that grow on the surface of wood. They don’t impact strength.” But they’re not pretty. Assuming you don’t want to simply paint the fence to cover up the stains, the greenest option would be to use plain old water and a pressure washer. Use the lowest setting; you don’t want to damage your fence.

Finally, given that this fungus is largely on the surface of the wood, you could resurface the fence, says Stirling—ideally, with a power sander. Sanding by hand would work, he says. “But that could be laborious.” 

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It’s also possible that you’re seeing the early stages of decay. It can mimic black stain, says Stirling. “But decay will turn the wood soft. Black stain won’t.” Cleaning won’t stop the wood from decaying. And “the cost of any remedial treatment would be more than the cost of the boards,” says Stirling. Ultimately, the fix would be to replace them.

Got a question for Cottage Q&A? Send it to answers@cottagelife.com.

This article was originally published in the March/April 2023 issue of Cottage Life.

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