Industrial Paint & Coatings Since 1916

Ask the Expert: Mold, Mildew, or Rot?

By Stacy Durr Albert:

There’s nothing that ruins the look of beautiful wood more than unsightly stains. But mold and mildew aren’t the only culprits when it comes to wood discoloration-harmless tannins and blue stains may also blemish your wood.

We asked Continental Products to explain:

What are tannins?

Many species of wood that are commonly used in log home construction contain tannins, which are naturally occurring water soluble chemicals in the wood that tend to migrate or bleed to the surface. This can cause discoloration to the coating, but it’s a cosmetic issue that doesn’t pose a threat to the wood or the coating.


What is a blue stain?

Also called sap stain, blue stain is by a species of fungi that attacks the natural sugars located in the sapwood of softwood lumber species. Species like spruce, fir, balsam, and cedar aren’t as susceptible to sap stain as pines.

Blue stain occurs in the wood, whereas mold and mildew occur on the wood or only slightly below the wood’s surface. Blue stain is also completely cosmetic and doesn’t impact the structure of the wood or the performance of any stains or sealants applied to the wood.


How do you determine whether your wood stain is caused by tannins, blue stain or mold/mildew?

A simple test is to apply a small amount of household bleach to a test patch with a cotton swab. If the blackened color goes away, chances are high that you have mold. If the stains aren’t impacted by the bleach, you’ve probably got tannin or blue stain.