Clear exterior finishes: finding the balance between aesthetics and durability

JCT CoatingsTech, Sept, 2004 by Tom Daniel, Marc S. Hirsch, Ken McClelland, Alan S. Ross, R. Sam Williams

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Discolorations

Moisture contributes to many common wood discolorations, such as extractive bleeding, iron stain, and mildew. These discolorations, although not harmful to the wood, give an objectionable appearance to the wood surface.

EXTRACTIVE BLEEDING: Certain types of lumber such as cedar, redwood, mahogany, and Douglas-fir contain water-soluble extractives that can be leached to the surface of the wood (Figure 4). As water moves through wood or as the surface of the wood gets wet, water-soluble extractives are dissolved. As the water evaporates, these extractives are deposited on the exterior surface and appear as a reddish brown stain. If the wood finish is not formulated to block the diffusion of these extractives, they can cause an unsightly residue on the finish, particularly with light-colored finishes. Extractive-related discolorations can be difficult to remove if they are allowed to remain on a surface for a long period of time. Although extractives in wood give the wood its natural color and resistance to decay (for some wood species), they must be sealed in the wood with a stain-blocking primer when using light-colored finishes. Extractive-related discolorations are usually not a serious problem when using dark stain or clear finishes.

IRON STAIN: Iron stain occurs in two ways on wood. Rust from iron or steel fasteners, railings, supports for window air conditioners, etc., can be washed down a structure by rain and absorbed by the wood or finished wood to cause brown discoloration. Iron can also react with the extractives in wood to give a blue-black stain. Although these stains can sometimes be removed by treatment with oxalic acid or sodium biflouride, it is best to avoid the problem by using corrosion resistant materials.

MILDEW: Airborne mildew spores are ubiquitous and microscopic and therefore are difficult to detect until they colonize a surface. They are transported easily by air currents, insects, and animals. These spores can cause mildew fungus growth on any surface that provides a food source. Food sources for mildew include wood extractives, plant and tree pollens, natural oils such as linseed and tung oils, bacteria, dirt, and other natural organic materials. Because mildew uses organic materials for food, it has a greater tendency to grow on organic surfaces such as wood, leather, and paper rather than inorganic surfaces such as aluminum, vinyl, and glass. Mildew can also grow on these substrates, but it must then depend on airborne food sources. Although extractives and natural oils give some species of wood their deep rich color, these organic compounds also make wood an ideal substrate for mildew growth, if there is enough moisture available (Figure 5). Therefore, because wood has readily available food for mildew growth, mildew grows more readily on bare wood surfaces than on painted wood, particularly wood painted with synthetic organic polymers such as acrylics.

Following colonization, mildew needs food, moisture, air (oxygen), and moderate temperatures [usually between 4.4[degrees]C and 32.2[degrees]C (40[degrees]F and 90[degrees]F)]. Because of the extremes of conditions in which it can flourish and the wide variety of species, mildew is very hard to control and impossible to totally eliminate. Of the requirements needed for mildew growth, moisture is the only one that can be controlled. It is impossible to control colonization, food source, temperature, or oxygen, and it is difficult to control moisture; therefore, paint and stain companies incorporate mildewcides in their products to retard mildew growth.