Extractives

The extractives in the heartwood of species such as western red cedar, cypress, and redwood give them their attractive color, good stability, and natural decay resistance, but they can also discolor paint. The heartwood of Douglas fir and southern yellow pine can also produce staining extractives, although the problem is not as severe as that encountered with western red cedar, cypress, and redwood.

When extractives discolor paint, moisture is usually the culprit. The extractives are dissolved and leached from the wood by water. The water then moves to the paint surface, evaporates, and leaves the extractives behind as a reddish-brown stain. The discoloration may be streaked or diffuse in appearance. Latex paints and the so-called breather or low-luster paints are more porous than conventional paints and thus more susceptible to extractive staining.

To stop extractive discoloration, any moisture problems must be eliminated. Streaked 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, discolored areas should be washed with a mild detergent soon after the problem develops. Paint cleaners may also be effective on darker stains. A bleach solution containing 4 to 16 oz. Of oxalic acid per gallon of warm water may also be effective. (WARNING: Oxalic acid and its water solution are toxic and should be used with the usual precautions.)

Iron Stains

Iron stains can also cause discoloration. Two types of stains are associated with iron. The red-brown discoloration caused by ordinary rust is associated with the use of ferrous nails and metal screens. The other stain, a blue-black discoloration, is caused by the reaction of wood extractives with iron and is associated with the use of ferrous nails and with iron traces from tools.

When standard ferrous nails are used on exterior siding and the siding is finished with paint or solid-color stain, a red-brown rust stain may appear through the finish in the immediate vicinity of the nail head. To prevent rust stains, use corrosion-resistant nails. These include high-quality hot dipped galvanized, stainless steel, and aluminum nails.

Blue-black stains can be caused by the chemical reaction of wood with iron. In this case, the iron reacts with certain wood extractives, such as tannins or tannic acid in cedar, redwood, or oak. Ferrous nails and other iron appendages are the most common source of iron in chemical staining.

A solution of oxalic acid in water will remove blue-black discoloration providing it is not already sealed beneath a finishing system. The solution usually contains ½ to 1 lb. of oxalic acid per gallon of hot water. After the stains disappear, the surface should be thoroughly washed with fresh warm water to remove the oxalic acid and any traces of the chemical causing the stain (see oxalic acid Warning described above.)