Naturally occurring chemical extractives in redwood are in large part responsible for its beauty, stability and durability; however, sometimes these water soluble extractives migrate to the wood’s surface. High concentrations cause a discoloration referred to as extractive bleeding. Extractive bleeding appears in several forms. Some boards, high in extractive content, may turn very dark if unprotected by finishes and exposed to moisture. Extractives may bleed through paints if stainblocking oil or alkyd primers are not used. In other cases, extractives may discolor the face of paints or other finishes if they are allowed to drip over them. Condensation on the unprimed back side or wicking under the lap from the face of bevel siding are common causes of this.
Oxalic acid, readily available in paint and hardware stores, is used to diagnose and remove extractive stains. A solution of oxalic acid crystals dissolved in water should be applied to a small discolored area. Extractive stains will fade and disappear in twenty minutes or less following this application. To completely remove the extractive stains, wash with one cup of TSP and one cup of household liquid bleach mixed in a gallon of water. Rinse thoroughly and follow with an application of four ounces of oxalic acid crystals dissolved in one gallon of warm water in a non-metallic container. Apply this to one entire board or surface area at a time with a soft brush. When the wood dries thoroughly, rinse with clean water.
CAUTION: Oxalic acid is poisonous but not dangerous if precautions are taken. We recommend that you hire a professional painter to work with this chemical. If you do the work yourself, wear rubber gloves and be careful not to let the acid or solution touch your skin or eyes. Wear old clothes. Try not to spill any on plants. When you have finished, wash containers, clothes and brushes thoroughly.