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 this is not a
mildew problem. If it is, scrubbing the wood with a solution of
one quart of bleach in a gallon of water — followed by a
clear-water rinse — should remove the stains.
Iron staining. Low-quality stainless steel nails, and
even the wire used to collate pneumatic-gun nails, can cause
staining. There is a simple test you can use to see if the
staining on the wood deck comes from iron contamination: Apply
a saturated solution of oxalic acid (typically used for
bleaching wood, especially oak) in warm water; if iron is the
source of the contamination, this solution will remove the gray
or black stains in a few minutes. If that is what happens,
brush the oxalic acid solution on the affected areas and then
on the entire deck. After the wood has dried, rinse the deck
thoroughly with clear water.
Water-soluble wood extractives. In some tree species
— including American or true mahogany, African mahogany,
and Philippine mahogany (meranti or lauan) — the
heartwood contains water-soluble extractives that give the wood
its attractive color. If exposed to enough moisture, these
extractives can dissolve and migrate to the surface of the
wood, leading to staining. Meanwhile, nails and screws that
penetrate wood expose end-grain along the shank of the
fastener. Because water enters and evaporates from end-grain so
readily, areas around the nail heads would have higher
concentrations of the extractives, leading to staining and
discoloration. These extractives are light-sensitive, often
darkening initially, then bleaching out with prolonged sun
exposure. If extractives are your culprit, washing the decking
with an oxalic-acid solution as described above might be all
that’s needed to fix the problem.