A.Bill Feist, a former wood-finishes researcher with the Forest Products Laboratory in Madison, Wis., responds: There are many different species and subspecies of mahogany, with varying properties, so it’s difficult to pinpoint your problem. But the staining is mostly likely due to mildew, iron in the nails, or extractives in the wood.

Mildew. A form of stain fungi, mildew is probably the most common cause of wood and house-paint discoloration. To test for the presence of mildew on wood, apply a drop or two of liquid household bleach (5 percent sodium hypochlorite) to the stain. The dark color of mildew will usually bleach out in one or two minutes — though my guess is that...

or Register to read the full article.