What is Sapstain – Blue Stain?

What is Sapstain – Blue Stain?


The term sapstain and blue stain is used in many articles concerning decay and wood preservation. One should understand this term as a first step in understanding wood degradation and decay.

What is Sapstain?

A tree consists of many parts, but for this short treatise two terms, heart wood (physiologically dead wood located near the center of the tree) and sap wood, which are located on the outer edge of the tree (physiologically active) will suffice.  The sap wood carries water and nutrients up though the tree and consequently is a perfect ecosystem for sapstain growth.

When a board or timber is cut from a log, mold spores can come in contact with the nutrient high wood with its inherent high moisture content and they begin to multiply and spread over the confront of the lumber.

As they spread they create a stain which is typically blue, but can truly be different colors. Throughout the years I have seen not only blue, but also black, red and yellow.

If this mold/stain is not checked, it can penetrate into the wood with tendrils and take the infection thorough into the lumber or timbers. They will ultimately release enzymes which will break down the wood and then you have the start of serious decay.  We are talking about lumber which is wet (unseasoned) and in a nutrient high ecosystem. Many people talk of dry decay; but in reality there is no such thing. This is a wrong name. since wood needs to be between 30 and 60-percent moisture content for it to begin the decay cycle.

Preventing Sapstain

To prevent sapstain from discoloring the lumber and to prevent further degradation by decay, many mills will dip the product into a fungicide to deter mold from attacking the lumber. This is particularly important in the humid areas of the United States and its use is highly recommended for high quality, valuable lumber.  

Lumber which has a high propensity of sapstain will not take a finish uniformly. The area which has the sap- stain will absorb wood finishes more than unaffected areas. consequently, when finishing a piece of lumber so infected, one will have to use a number of wood finish coats to already out the look. For high quality products such as furniture or mouldings, the lumber infected with sapstain is discarded.  

If one is installing wood which has mildew present, it should be washed thoroughly before applying any kind of finish. We are talking about a surface coating of mildew, not a stain that has penetrated thorough into the wood. Use a combination of three quarts of water, one quart of household bleach, and 1/4 cup of liquid dishwasher detergent which does not contain ammonia. Never mix bleach and ammonia as it will give off hazardous fumes. This formulation can be sprayed on using a pump sprayer. After 15 to 30 minutes, it can be washed off using a strength washer.

Remember that once the stain goes thorough into the wood, it cannot be removed with bleach or any other product. It is there to stay.  Always remember not to play chemist and experiment by mixing various chemicals as they could give off hazardous fumes…and more!

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