Staining Previously Painted Cedar Siding: Instructions on a Tricky Job

Solid-Color Stains

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Severely weathered wood surface.

When reapplying solid-color stain, wash the surface to remove dirt and mold. If areas have peeled exposing the wood surface, the weathered surface must be removed prior to re-priming the peeled area. The photo shows an extreme case of surface degradation following the failure of the coating. Solid-color stains form films, and like all other film-forming finishes, do not bond properly to weathered wood. It has been well established through several studies that cedar exposed to sunlight for as little as 2 to 3 weeks will not hold film-forming finishes as well as an unweathered surface. It may be possible to remove the damaged wood from small areas by scuff sanding, but this is difficult with shakes and shingles because the surfaces are saw-textured or split. The sanded surface won’t match the unsanded surface. If the previous finish contained lead, do not sand.

The situation shown in the photo is an extremely difficult problem to reconcile. A penetrating finish, such as an oil-based semitransparent stain, cannot be used over the existing finish, and a film-forming finish will not adhere to the weathered surface. It is probably necessary to remove all the finish and the weathered surface by power washing prior to application of the primer and solid-color stain. This may be one of the few instances where careful power washing by a skilled operator is necessary. As with using a garden hose to wash siding, keep the nozzle pointed down to avoid forcing water behind the siding. Paints refinish in the same way as solid-color stains. Lap marks should not be a problem with paint systems.

Careful and judicious work will yield good results for the homeowner.

 

Cleaning and Staining Finished Surfaces: More DIY for Cedar Siding

What about do-it yourselfers who want to work with cedar siding or shingles that have already been finished? The FPL report, Installation, care, and maintenance of wood shake and shingle siding, can help with that.

Finished Surfacesshutterstock_56678470

Finished surfaces (paints, semitransparent stains, and solid-color stains) often develop mold and algae. As with wood surfaces, these microorganisms feed off airborne contaminates, but can also metabolize oils in some finishes. Surfaces of paint and solid-color stain can easily be cleaned with commercial cleaners or a dilute solution of household bleach and as with wood, the gentler the better. Abrasive methods will damage the finish surface. It is more difficult to remove these microorganisms from saw-textured wood; the microorganisms may be inter-grown through and under the finish.   

Refinishing

Wood Repellant Preservatives (WRPs) and Semitransparent Stains

Other than mold removal and gentle washing, WRP and semitransparent stains should not require additional surface preparation. They can be reapplied as soon as shakes or shingles are dry following cleaning. The reapplication of semitransparent stains and tinted WRPs requires special care. Usually residual finish is left in some areas, whereas the finish may be completely weathered away in other areas. The weathered and unweathered surfaces accept penetrating stains differently; the weathered areas absorb stain easily, whereas stain forms a second coat on areas having residual stain. During application, feather the stain into areas having old finish to avoid forming this second coat. If a second coat is formed, it will give a shiny appearance and will fail by flaking. When refinishing a structure, it is often necessary to have the north, east, west, and south sides on different cycles. Semitransparent stains usually last 4 to 5 years on the south side and more than 10 years on the north side. These estimates can vary considerably depending on the design of the structure, exposure conditions, and the quality of the shake or shingle. As with the application for new construction, take care to avoid lap marks. Changing colors or switching to a paint or solid-color stain is not an option with semitransparent stains unless the old finish and weathered surface is removed. The surface must be scuff-sanded, primed, and finished (at least two coats may be required to hide residual stain).

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.

House_Stained-siding

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.

 

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.

Finishes for Cedar Siding: Finish Selection

Installation, care, and maintenance of wood shake and shingle siding instructs us in the proper finish to apply to cedar siding. Wide selections of oil- and latex-based finishes are available for shakes and shingles. Oil-based finishes are solvent-borne, and latex-based are water-borne; however, in recent years the solvent-borne finishes often have co-solvents in them to make them water soluble. As with any other wood product, shakes and shingles can be finished with water-repellent preservatives (WRP) (clear and lightly pigmented), semitransparent stains, solid-color stains, or paints.

Water-Repellent Preservatives

Water-repellant preservative is a generic term that describes a clear penetrating finish that traditionally was formulated with about 10% to 20% oil or alkyd binder, 1% to 3% wax or similar water repellent, a preservative, and an organic solvent such as mineral spirits or turpentine. The finish does not contain a pigment and gives a natural wood appearance.

Figure-5.-Newly-installed-shingle

Newly installed shingle siding with transparent water-repellent preservative finish.

The low wax content in some of the traditional WRP formulations made it possible to use them for pretreatment of wood prior to painting. They were also an excellent pretreatment for shakes and shingles prior to installation. Bundles of shingles could be dipped. Most of the finish was absorbed into the end grain at the butt end of the shake or shingle and gave years of protection by minimizing end-grain water penetration.

To meet stringent air quality requirements, these solvent-borne types of WRPs are no longer available. They have been largely replaced by water-borne formulations and formulations having low organic solvents content. Many of these formulations are intended for use on wood decks, but they can also be used on siding. These finishes often are tinted with a small amount of pigments, UV stabilizers, and other additives to improve their service life.

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Shingles treated with tinted water-repellent preservative.

They give some water repellency and inhibit mold and algae growth on the lateral surface. If used on single-story structures having wide roof overhang, they give about 2 to 3 years of service, depending on exposure.