If discoloration is to be stopped, moisture problems must be eliminated. Run down discoloration will usually weather away in a few months. However, discoloration in protected areas can become darker and more difficult to remove with time. In these cases, wash the discolored areas with a mild detergent soon after the problem develops. Paint cleaners are effective on darker stains.
Blue stain is caused by microscopic fungi that commonly infect the sapwood of all woody species.
Although microscopic, they produce a blue-black discoloration of the wood. Blue stain does not weaken wood structurally, but conditions which favor stain development are also ideal for serious wood decay and paint failure. Wood in service may contain blue stain, and no detrimental effects will result so long as the moisture content is kept below 20 percent. Wood in properly designed and well maintained houses usually has a moisture content of 8-13 percent. However, if the wood is exposed to moisture such as rain, condensation or leaking plumbing, the moisture content will increase, and the blue stain fungi will develop.