Moldy Wood Is Wet Wood
When you see mold growing, you know moisture is present, but it isn't always caused by a visible leak. The mold could be from high humidity, condensation or even moisture in the wood itself.
Lumber that has come fresh from the lumberyard may be slightly green. As the natural moisture leaches to the surface, it feeds the mold microorganisms that are present in the air virtually everywhere, so they begin to grow.
You have to clean the mold before sealing the wood, but if the wood doesn't dry first, the mold will return; the moisture trapped under the seal coat may cause warping.
Wet Down Powdery Mold
Even though the mold on unfinished wood is feeding on moisture, it's often powdery. If you simply brush it off, you'll release the spores into the air, and you can inhale them and subject yourself and your family to respiratory problems, an allergic reaction or worse.
To prevent an accidental release, spray any mold with an appropriate cleaning solution before disturbing it by scrubbing or brushing. A mixture of 1 part bleach to 10 parts water kills the mold, but a mild detergent solution is just as effective.
Wear rubber gloves and a respirator while performing any mold remediation for safety.
Scrub Off the Mold
No matter how much bleach or other disinfectant you spray on mold spores, you won't remove them; instead, you must scrub the area to get rid of the mold. Once you've saturated the mold with your cleaning solution, scrub off all the discoloration with an abrasive sponge.
If you're cleaning severely blackened wood, you may need a strong cleaning solution; the US. Forest Products Laboratory recommends a mixture consisting of 1 part detergent, 10 parts bleach and 20 parts warm water.
Use a toothbrush to reach into tight corners.
Preventing Mold From Growing
Once you've cleaned off the mold, the wood needs to dry, and it needs efficient air circulation to do so. Use a fan, if necessary, and if the humidity in the room is high, run a dehumidifier to reduce it.
You can prevent mold from returning by ensuring the humidity remains low and the air keeps moving. If you're storing lumber, stack it in layers so that all sides of the wood can breathe.
Once the mold remediation is complete and the wood dries out, some discoloration may remain. You can usually remove that by sanding.