John Leeke's Historic HomeWorks™

    207 773-2306      26 Higgins St. Portland, ME  04103
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"Cleaners And Restorers For Wood Decks And Siding"
Joint Coatings/Forest Products Committee Chairman: Alan Ross, Kop-Coat, Inc., Pittsburgh, PA (1-412-826-3387) George Daisey, Rohm & Haas Company, Springhouse, PA Charles Jourdain, California Redwood Association, Novato, CA Sam Williams, USDA Forest Products Laboratory, Madison, WI

Introduction..........(Return to Table of Contents)

The popularity of wood decks in the United States remains at an all-time high. It is estimated that one out of every four residential dwellings in this country has a wood deck and that they are being added to existing homes at a rate of over a million per year.

Most homeowners and painting contractors recognize the advantages of coating decks with clear or pigmented finished to beautify and protect the wood's surface. As the surface is exposed to sunlight, dirt, mildew, rain, or snow it will eventually deteriorate in appearance. The phenomenon occurs fairly rapidly to uncoated wood. It can be prolonged from occurring by the use of a protective finish; however, even the best finish will succumb to the ravages of weathering in a few years and will need to be refinished. Proper cleaning and restoration is an essential first step in the refinishing of wood decks.

There are a variety of products and methods to clean and restore wood surfaces. These include chemical as well as mechanical means. This paper will review some of the types of deck cleaning and restoring products currently available and will also discuss the pros and cons of mechanical cleaning methods such as power washing. Although this paper is focusing on wood decks as the primary substrate, most of the principles discussed here hold for wood siding, as well. The section on VERTICAL SURFACES addresses some cleaning/restoring situations which are specific to siding applications.

Causes of Discoloration..........(Return to Table of Contents)

There are a number of sources of discoloration of wood decks. These include:

Some of these discolorations are chemical in nature; others biological. All require some effort on the part of the homeowner or contractor for removal and all should be removed prior to refinishing. In addition to being unsightly, these discolorations and the agents that cause them can significantly interfere with the performance of any subsequently applied coatings. Thus, their removal is important from a performance as well as an aesthetic standpoint.

Deck Cleaners and Restorers..........(Return to Table of Contents)

Until about ten years ago there were few, if any, products of this type on the market. Most coatings manufacturers recommended that decks and other exposed wood surfaces be cleaned before refinishing. The usual recommended cleaners were household products such as detergents for dirt removal and liquid bleach for mildew removal.

Household cleaners and bleaches can be effective to some extent but they have their limitations. Also, since they are not usually designed for deck cleaning applications they can present some handling problems to do-it-yourselfers and contractors. For example, liquid household bleach should not be mixed or used directly with ammonia or any other detergents or cleaners containing ammonia since the resulting chemical reaction can form a potentially dangerous gas.

About ten years ago products began appearing in the market that were specifically designed to clean and restore weathered wood surfaces such as decks and siding. Today there are a variety of such products available. Deck cleaners and restorers generally fall into one of three categories--chlorine bleaches, oxygen bleaches, or oxalic acid-based formulas. Each of these is discussed below.

Chlorine-Based Bleaches
Common types of chlorine bleach used in deck cleaning products are sodium hypochlorite, calcium hypochlorite and dichloroisocyanurate. The first two are typically used in laundry detergents while the last is a swimming pool additive. These chemicals are effective against mildew but do little to remove dirt or other surface deposits (which is why bleach alone does not get clothes clean). When used on wood decks, chlorine-based bleached products can do more harm than good. They can result in the wood's having a whitish unnatural tone due to the bleaching of natural components or a fuzzing of the wood's surface due to the loosening of small fibers during the cleaning process. Moreover, if not rinsed properly, the chlorine salt residues can result in premature graying of wood from the action of sunlight.

As noted previously, household bleach and other products containing chlorine-based bleaches should not be mixed with
products containing ammonia.

Oxygen-Type Bleaches
Products in this category are usually based on disodium peroxydicarbonate, commonly known as sodium percarbonate, an ingredient present in some color safe fabric bleaches. Sodium percarbonate is a powder. When added to water it forms hydrogen peroxide--a common oxygen bleach--and sodium carbonate (soda ash). Hydrogen peroxide is commonly used as a disinfectant and a stripper of hair coloring. On wood it is effective in removing mildew stains and weathered gray residue from UV (sunlight) degradation. The sodium carbonate acts as a built-in detergent, removing dirt and other deposits. Thus, sodium percarbonate-based cleaners are effective in removing dirt, mildew and weathered gray residues. Once treated, the wood returns to its natural original appearance.

Oxalic-Based Products
Certain wood species such as cedar and redwood contain natural resins known as tannins. These are water soluble materials which are reddish brown in color. Water can extract the resins from within the wood and deposit them on the surface, leaving brown or black discolorations. Tannins can also react with iron present in fasteners or nails resulting in blue-black stains. Neither chlorine bleaches nor oxygen bleaches are effective against tannin stains or iron stains. Oxalic acid, on the other hand, solubilizes tannins and iron stains and renders them colorless. Thus, it is the material of choice for use on redwood or cedar. However, oxalic acid is not as effective against mildew. For this reason some homeowners and contractors will treat redwood and cedar with a sodium percarbonate or chlorine-based cleaner and follow it up with an oxalic acid-based product if tannin staining is apparent. Concentrating oxalic acid is toxic and should be handled and used with car.

Paint Strippers..........(Return to Table of Contents)

Sometimes, in order to restore a wood deck and prepare it for refinishing, the previous finish, or what's left of it, will need to be removed. Most deck cleaners and restorers are not effective in removing paints and stains. This can be accomplished by mechanical removal or chemical means. Mechanical removal will be discussed in the next section.

Chemical paint strippers are usually based on organic solvents, caustic salts such as sodium hydroxide or sodium metasilicate. Most of these strippers are pretty potent and require some care in use and handling. Contact with skin or eyes must be avoided. Proper protective clothing and equipment must be worn as outlined on the product labels. Depending on the formulas, strippers will remove varnishes, oil-based stains or latex stains and paints. Most are supplied as ready-to-use liquids.

Residual amounts of the stripper should be thoroughly rinsed from the wood before any product is re-applied since residual traces of caustic salts can interfere with subsequently applied coatings.

Mechanical Cleaning..........(Return to Table of Contents)

Mechanical methods for cleaning and restoring the surface of wood decks include planing, sanding, and power washing. Planing removes the outer surface of the wood face exposing fresh, new wood. Decking boards have to be removed from the structure and passed through a planer to accomplish this. Planing is very effective but its usage is limited by the need to physically disassemble part of the deck to carry it out. Since planing removes the outer veneer of wood it also results in a change in physical dimensions of each board.

Sanding is effective as a mechanical means for removing unwanted coats of previously applied finishes. It can, however, damage the surface of the wood. On redwood and cedar sanding often results in an excessive amount of tannin resin bleeding.

Power washing is the mechanical method for cleaning and restoring decks most favored by contractors. Power washers direct a pressure jet of water at the wood surface. This pressurized water is effective in removing dirt, mildew, algae and gray weathered residue from most wood surfaces. It can also be effective in removing previously applied coatings. Some contractors have found that the best cleaning procedure is to treat with a chemical cleaner and follow up with a wash/rinse from a power washer.

Power washers are available to homeowners through purchase or rent from many paint or home center stores. First time users need to be cautioned since excessive pressure can damage wood deck surfaces. If not used properly power washers can also cause damage to windows, doors and siding. For this reason many do-it-yourselfers prefer to stick to chemically based means of cleaning and restoring their decks.

Refinishing..........(Return to Table of Contents)

As noted previously, proper surface preparation of weathered decks is an essential first step to the successful refinishing of these substrates. Failure to remove dirt, mildew and weathered residues is an open invitation for early failure of subsequently applied coatings.

Care should be taken to thoroughly rinse all cleaner/restorer products from the wood. In addition, many coatings require all dry surfaces prior to application, so contractors and homeowners should allow adequate time for the wood to dry before applying a finish.

Vertical Deck Surfaces and Siding..........(Return to Table of Contents)

Most decks contain some vertical components such as rails and spindles, and these require the same degree of cleaning and restoring as do the horizontal planks which make up most of the deck surface. Products designed to clean horizontal surfaces will generally work on vertical facings as well. The only limitation for some of these products is a tendency to run off the vertical components before active ingredients can fully function. To address this problem, some manufacturers have formulated their cleaners/restorers/strippers with thickening agents to help the liquids cling better to vertical surfaces. This improves performance since it increases the contact between the active ingredients and the surface being treated. Users should consult product labels to determine whether or not the formula contains thickeners.

Although this paper is focusing on wood decks as the primary substrate, many of the principles discussed here are appropriate to siding applications as well. Since the majority of siding is in the vertical orientation, it is especially helpful to utilize a product, which is designed for use on vertical facings, as discussed above. In addition to deck cleaners, restorers and strippers, there are products available which are specifically targeted for use on wood siding. Manufacturers' labels should be consulted for appropriate uses and directions for use.

Conclusion/Summary..........(Return to Table of Contents)

Contractors and homeowners now have a number of choices in cleaning and restoring weathered wood decks prior to refinishing. Whereas household bleaches and detergents were formerly used for this purpose with minimal success, a variety of products are now available which are specifically designed for this purpose.

Formulas based on chlorine bleaches are effective against mildew but have limited efficacy against other sources of discolorization. Products based on sodium percarbonate are more effective against a wider range of discoloring agents and leave the wood with a more natural appearance than do chlorine bleaches. Oxalic acid-based cleaners are particularly effective against tannin and iron stains and find most use on cedar and redwood.

The mechanical method of choice for cleaning and restoring wood decks is the power washer. This is sometimes used in conjunction with chemical cleaners. Power washers are used more by contractors than do-it-yourselfers since they are somewhat tricky to use and can damage the wood if not utilized properly.

Paint strippers are heavy duty products designed to chemically remove unwanted coats of paint from wood surfaces. They are effective but need to be used with caution since they may contain hazardous ingredients.
Products that are formulated with thickeners will cling better to vertical deck surfaces such as rails and spindles. These may be suitable for wood siding, as well.

Proper cleaning and surface preparation is essential to successful refinishing. Without this step, early failure of refinishing is unlikely.

Bibliography..........(Return to Table of Contents)

Fantozzi, J., Bussjaeger, S., Carlson, R., Donegon, V., Feist, W.C. and Wagner, L., "Surface Preparation and and Finishing of Exterior Wood Surfaces," The Paint Dealer,Sept. 1995 (pp-94).

McDonald, K.A., Flak, R.H., Williams, R.S. and Winandy, J.E., "Wood Decks: Materials, Construction and Finishing," Forest Products Society, Madison, WI, 1996.

Ross, A.S., Bussjaeger, S., Carlson, R. and Fiest, W.C., "Professional Finishing of CCA Pressure-Treated
Wood," American Painting Contractor,69(7), 1992 (pp17-114).

Ross, A.S., "Avoiding Misconceptions About Finishing Wood Decks," American Painting Contractor,71(7), 1994.

Williams, R.S., Knaebe, M.T. and Feist, W.C., "Finishes For Exterior Wood: Selection, Application,
Maintenance," Forest Products Society, Madison, WI, 1996.


Webpage creation by Dan Mitchell, working in the Historic Home Works Assistantship Program. Dan is a preservation enthusiast on Cobbosseeconte Lake in Maine.
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John Leeke's Historic HomeWorks™

    207 773-2306      26 Higgins St. Portland, ME  04103
[Home][Library][Restoration Reports][Seminars][Forum][Internships][Office][Workshop][Front Porch][Search] info.© 1994-2007 JohnC.Leeke