Removal of Algae, Mold, and Iron Stains
As wood ages, mildew (mold) and algae begin to grow on the surface. This is a normal process and these organisms do not degrade the wood; they cannot break down the structural components of wood. Although they can metabolize extractives in wood, they primarily feed off airborne contaminants. Because they do not attack the wood, removing them is a cosmetic issue: ignoring them will not damage the wood. Algae and molds can be cleaned quite easily and effectively with bleaching agents such as sodium hypochlori
We recommend using commercial cleaners containing sodium percarbonate or other oxygen bleaches because they are more gentle oxidizers than chlorine-containing bleaches such as household bleach. Chlorine bleaches may cause excessive pulping of the wood to give a fuzzy surface. Some commercial cleaners contain strong alkali (sodium hydroxide or potassium hydroxide). These ingredients help to remove residual finishes on the wood surface, but can cause even more surface damage than chlorine bleach.
When using a cleaner, you might mix the solution weaker than the recommended strength and try it on a small area. If the weaker solution doesn’t work, increase the concentration until you find a concentration that cleans the wood. A fuzzy surface appearance or excessive removal of surface fibers indicates that the solution concentration is too strong. Apply cleaning solutions with a garden-type sprayer, sponge mop, or soft bristle brush and keep the surface wet with the cleaning solution for 12 to 15 min. I
We recommend using commercial cleaners, but if you prefer to use liquid household bleach, start with a cleaning solution having about five parts warm water, one part bleach, and a small amount of powdered detergent. Do not use detergent that contains ammonia. Ammonia reacts with bleach to form toxic fumes. If the surface mildew is difficult to remove, try using a stronger solution of three or four parts water added to one part bleach and detergent. It should not be necessary to use a concentration stronger
If the dilute cleaning solutions described above are not effective, it is probably because the mildew has inter-grown with residual finish on the surface. The residual finish is keeping the cleaning solution away from the mildew. In this situation, it may be necessary to use aggressive cleaning methods, such as the cleaners containing strong alkali. In extreme cases, it may be necessary to use a paint stripper to remove the residual finish prior to cleaning.
As with unfinished wood, wood finished with WRPs and semitransparent stains degrades as these finishes degrade. Ultraviolet radiation in sunlight degrades lignin at the surface. Lignin is the natural glue in wood that holds the cellulose fibers in place. Degradation of lignin weakens the surface fibers, and strong cleaning solutions and aggressive methods will remove excessive amounts of fiber from the surface. Removing these fibers is detrimental to the performance of subsequent application of WRPs or sem
Iron reacts with the some of the extractives in cedar and redwood to give a dark blue-black stain. This often occurs when the zinc on galvanized fasteners weathers away. Use only hot-dipped galvanized fasteners that meet ASTM A153/D or type 304 or 316 stainless steel that meet ASTM A276. Shakes and shingles can also be contaminated with rust washed from other sources such as window screens, failed flashing, or metal ornaments. A 5% solution of oxalic acid (usually available at drugstores) neutralizes this b
Note: Oxalic acid is toxic. Many commercial wood cleaners contain oxalic acid, which neutralizes iron stains and removes extractive stains. Oxalic acid generally brightens wood surfaces, but is not very effective for removing algae or mold, nor will it keep iron stain from reoccurring if the source of the iron is not removed.
It is best to apply cleaners on a cloudy day. This slows evaporation and allows the cleaner to work more effectively. Wear eye protection, rubber gloves, an apron, boots, and other protective gear when using and preparing cleaning solutions. Avoid getting cleaning solutions on yourself, other parts of the structure, and vegetation. If the cleaning solution comes in contact with you or these other areas, rinse promptly and thoroughly with water. Follow all additional instructions on labels and technical data
In the rare case that pressure washing of shakes and shingles is necessary, it should be done only by contractors having experience cleaning cedar. In the wrong hands, high-pressure water can easily remove an excessive amount of wood fiber, which could drastically decrease the life expectancy of your shakes and shingles.
Source: MaintainX (Community Member)