Specifiers: Why Does Wood Bleed?

Bleeding WoodColored wood such as cedar and redwood, and in some cases other common softwoods, can show a condition known as "cedar bleed" or "cedar staining." This can appear anytime from shortly after painting to months later. As the name implies, the most commonly seen staining problems involve the various types of cedar woods. These woods are very common and have good machining properties and are found used as structural members, siding, roofing, frames and trim for both exterior and interior applications.

Cedar is well known for it's durability as an exterior wood and it's resistance to insect and fungus attack. This is generally attributed to the presence of tannins and other substituted phenol compounds in the wood. Unfortunately these are also the compounds that contribute to the cedar bleeding. These materials are partially water soluble and become more soluble in alkaline water such as water that runs over a masonry surface onto partially or un-coated cedar trim.

In the past, oil based primers and paints made with a white lead pigment were used to block the appearance of cedar stains. Later, the use of latex emulsion type binders with white lead pigments created "blister-proof" primers and finishes. The concerns for human health and environmental damage stopped, in large, the use of these types of coatings. Newer systems are available in both solvent and water based types incorporate non or low toxicity type stain blocking pigments. A more recent development has been the stain resistant water based emulsion type polymers that do not require special pigments.

The difficulty has been to develop primers that will work under all application and exposure conditions. Wet or damp wood, low film build, contact with masonry, lack of back priming, leaky siding laps and humid conditions during painting are all negative factors.


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