Mahogany decking sometimes will develop dark staining around stainless steel nail heads. Since there are many different species and subspecies of mahogany, it's not always easy to figure out the cause of the problem or how to fix it. But as wood finishes researcher Bill Feist at the USDA's Forest Products Laboratory explains in this JLC Q&A, stains around stainless steel fasteners are usually caused by mildew, iron in the nails, or extractives in the wood. In each case, there's a straightforward way to diagnose what is causing the staining, and a simple cure.

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