Maintenance of Cedar Siding: Removal of Algae, Mold, and Iron Stains

Wood surfaces

As wood ages, mildew (mold) and algae begin to grow on the surface. This is a normal process; these organisms do not degrade the wood. They cannot break down the structural components of wood. They just live there. They feed off airborne contaminants, extractives, and oils in wood and in some finishes. Algae and molds can be cleaned quite easily and effectively with bleaching agents such as sodium hypochlorite (liquid household bleach) and sodium percarbonate (the active ingredient in some commercial cleaners). Bleaching agents quickly kill mold and algae, but they also can degrade wood. Therefore, mix cleaners as dilutely as possible. The object is to remove the fungi without excessive wood damage.

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The cedar siding on this house has become stained. Photo from Flickr: https://www.flickr.com/photos/jahluka/

The authors of the study, Installation, Care, and Maintenance of Wood Shake and Shingle Siding, 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 tend to cause excessive pulping of the wood to give a fuzzy surface. However, 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. The photo below shows a person cleaning a deck with oxygen bleach and a gentle brush. These same techniques can be used to clean shingles and siding.

wash-deck

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 minutes. It is best to work on a cool cloudy day or even during a gentle rain so the solution doesn’t evaporate. Aggressive scrubbing shouldn’t be necessary; let the cleaner do the work. Rinse with a garden hose keeping the water-stream pointed down. High pressure shouldn’t be necessary. You do not need a pressure washer! Allow the surface to dry for several days before refinishing.

We recommended using commercial cleaners, but if you prefer to use liquid household bleach, start with a cleaning solution of about five parts warm water mixed with one part bleach) with 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, you should then work with 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 than three to one.

If the dilute cleaning solutions described above are not effective, it is probably because the mildew is 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 more aggressive cleaning methods, such as the cleaners containing strong alkali.  In some cases, use a paint stripper to remove the residual finish prior to cleaning.

As with unfinished wood, wood finished with wood repellant preservatives (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 semitransparent stains. These finishes perform best by penetrating the wood surface and as a wood surface degrades, it becomes more porous. If excessive amounts of fiber are removed during cleaning, the surface will not accept these finishes as well as the porous weathered surface. Again, when cleaning wood, the gentler the better.

Iron reacts with the extractives in cedar redwood to give a dark blue-black stain. This often occurs when the zinc on galvanized fasteners weathers away or from rust washed from other sources such as window screens, failed flashing, or metal ornaments. This blue-black stain can be neutralized with a 5% solution of oxalic acid (usually available at drugstores).

Note: Oxalic acid is toxic. Many commercial wood cleaners contain oxalic acid. Oxalic acid will neutralize the iron stains and will also remove extractive stains. Oxalic acid generally brightens the wood surface, 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.

 

Wood Preservatives: New Report Explores Directions and Possibilities

A new report has just been published: Wood Protection Research Council, Research Priorities 2013

In this report, authors Carol A. Clausen, Frederick Green III, Grant T. Kirker, and Stan T. Lebow report on findings and recommendations from the Wood Protection Research Council.

Why wouldn’t a homeowner want to build with wood? Sometimes homeowners do not select wood as a building material because of its vulnerability to biodeterioration by fungi and insects under certain conditions of storage and use. These limitations are also a prime cause of user dissatisfaction. Therefore, efforts to protect wood from biological degradation are among the earliest research at the Forest Products Laboratory. This research has successfully reduced the demand for lumber from our National Forests by reducing the need to repeatedly replace existing wood products.

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The cycle of wood harvest, research, and use protects our natural resources.

Wood protection has undergone dynamic changes since the industry voluntarily withdrew chromated copper arsenate (CCA) from most residential uses and new products were introduced to the marketplace. According to the Environmental Protection Agency (EPA), “CCA is a chemical wood preservative containing chromium, copper and arsenic. CCA is used in pressure treated wood to protect wood from rotting due to insects and microbial agents. EPA has classified CCA as a restricted use product, for use only by certified pesticide applicators.”

Obviously, alternatives for wood protection are needed. However, to bring a new preservative to the marketplace, a considerable amount of performance data needs to be obtained. Current laboratory methodologies to determine the durability of test specimens are insufficient, and long-term field testing is required to ensure that a treatment is effective.

Improved accelerated test methods to predict performance would reduce the time needed for the development and acceptance of new preservatives. Potential improvements for accelerated testing may include selection of test fungi, techniques to detect incipient stages of fungal decay, methods to properly assess durability of wood plastic composites use of rapid laboratory bioassays for screening, and field tests that could measure loss in mechanical properties and statistical analysis.

Possibilities and research opportunities abound. For instance, protection systems could be targeted to specific problems. With nanotechnology at the forefront, novel advances in wood protection could replace the broad spectrum biocides traditionally used to inhibit decay fungi. The most logical approach to develop targeted biocides is to take advantage of unique physiological attributes of decay fungi, such as their ability to sequester metals through production of oxalic acid or natural tolerance to preservatives. Discerning and describing these mechanisms may enable us to design specific, targeted inhibitors to control decay and circumvent preservative tolerances that are common in brown-rot basidiomycetes.

This report summarizes presentations and comments from the inaugural Wood Protection Research Council meeting. Research needs for the wood protection industry were iden­tified and prioritized. Methods for successfully addressing research needs were discussed by industry, academia, and association representatives.

 

 

 

Cedar Siding: The Final Word on Finishing

Solid-Color Stains

Installation, Care, and Maintenance of Wood Shake and Shingle Siding tells the homeowner how to finish cedar siding. Solid-color stains (also called solid-body stains or opaque stains) are available in oil- and latex-based formulations.They give more protection against UV radiation than semitransparent stains. However, solid-color stains do not penetrate wood; they form a film just as paints do. To get good service life, they need to have sufficient film thickness to avoid failing by flaking and peeling. Prior to application, it is best to prime shakes or shingles with a stain-blocking primer prior to applying a solid-color stain to avoid extractives bleed. These stains perform best on textured or rough surfaces, but should not be used on a weathered surface.

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Single coat of solid-color stain. Note the extractives bleed.

Paint

Paint is a film-forming opaque finish and provides the best surface protection against water and weathering. Good quality stain-blocking primers applied to all surfaces prior to installation offer the best protection against discoloration by water-soluble extractives. (See discussion of back priming in Factory Finishing section.) Top coat with good quality acrylic latex-based paint. As with solid-color stains, paints will not adhere to weathered surfaces.

Caution: The use of transparent film-forming finishes such as urethane varnish, lacquer, conventional varnish, and shellac are not recommended for exterior use. Ultraviolet (UV) radiation can penetrate these types of finishes and degrade the wood. The finish will become brittle, then develop cracks and peel from the surface. Paint manufacturers continue to improve the UV radiation resistance of these products. It may be possible to get 2 to 3 years of good performance with finishes formulated with some of these new products, particularly on structures having moderate protection from sunlight (single-story structures having wide roof overhangs and/or shaded by trees).

Cedar Siding: Finish Application

Factory Finishing

Factory-finished shakes and shingles are available and in recent years have become the preferred product in many areas of the country. Installation, Care, and Maintenance of Wood Shake and Shingle Siding outlines how factory finishing offers several advantages over on-site finishing.

Factory finishing eliminates weather limitations and avoids damage by UV radiation to the unpainted wood. Back priming (the application of finish to the back side of the shake) of shakes and shingles is easily done during factory application. Back priming decreases water absorption and extractives bleed. The primer should extend about half to two thirds up the back side of the shingle. If pre-finished shakes or shingles need to be trimmed during application, the cut surfaces should be touched up with finish.

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An example of a lap mark from improper application of a semitransparent stain.

Onsite Finishing

It is possible to finish shakes and shingles prior to installation. It is a rather labor-intensive process, but will improve the performance of finishes on shake and shingle siding, particularly on structures having minimal roof overhang and on the sides of structures exposed to strong wind-blown rain. Finishing shakes or shingles prior to installation makes it possible to back-prime with a stain-blocking primer or penetrating stain.

The most effective means for applying finishes to wood is by brush. Brushes are an investment and it is wise to purchase top quality brushes, as they will last for a long time if properly cared for. For latex-based finish, use a synthetic bristle brush; for oil-based finishes, use a natural-bristle brush. For best results, purchase long-bristle brushes. If you choose to apply the finish by spraying or with a roller, brush the surface (back brush) immediately after application. Back brushing is essential to ensure that the finish is spread evenly and worked into the wood surface. Ensure that the butt-end gets a liberal application of finish; this is the most important surface. This is especially important for semitransparent stains and tinted WRPs. If using a roller, use natural roller covers for oil-based finishes and synthetic rollers for latex-based finishes.

For tinted WRPs and semitransparent stain application to installed shakes and shingles, it is necessary to take care to prevent lap marks (an area with two coats rather than one). Apply the finish in a single direction, usually across the structure, from a corner to a door, window, or other corner, keeping a wet edge throughout the application of this section. The lateral edges of the advancing strip of finish must coincide with the top and bottom edge of the course (s) of shake or shingle. When applying finish adjacent to the course that is already finished (the next strip), take care not to apply additional finish to the area that is already finished, as this causes a lap mark. For penetrating finishes such as WRPs, semitransparent stains, and to some extent solid color stains, lap marks give an unsightly blotchy appearance.

Semitransparent Stains on Cedar Siding: Another Option for the Homeowner

The favorite FPL report, Installation, Care, and Maintenance of Wood Shake and Shingle Siding, discusses semitransparent penetrating oil-based stains. These stains are similar to water-repellant preservatives (WRPs), but have considerably more pigments than WRPs.

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Semitransparent stain application to shingles that were let weather for about two years. NOTE: The stain application was not done correctly. The painter allowed stain to partially coat the bottom of the course of shingles directly above the stained portion. When the shingles above are stained, there will be a lap mark.

As with WRPs, new semitransparent finishes are low in organic solvents and may not penetrate wood as well as traditional semitransparent stains. Semitransparent stain provides a color without entirely concealing the color, grain, and texture of the wood. Stains with the highest pigment concentration will provide the longest service life and provide the most protection from weathering. Some companies formulate stains especially for use on wood shakes and shingles. (Check the internet for “wood shingles and paint” to find leads on paint manufacturers having products for wood shakes and shingles.) Stains that contain a water repellant and wood preservative will provide additional protection and further extend the service life of your siding.

Oil-based stains penetrate the wood and generally give longer service life than latex semitransparent stains do. Oil-based formulations degrade by slow erosion of the pigment from the surface and it is seldom necessary to do extensive surface preparation prior to refinishing. Latex semitransparent stains do not penetrate the wood but form a thin film on the surface. They tend to fail by flaking and may require extensive surface preparation prior to refinishing. Paint manufacturers and resin suppliers are making progress to overcome these limitations.