John Leeke's Historic HomeWorks™
	207 773-2306      26 Higgins St. Portland, ME  04103
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"Finishes for Exterior Wood Surfaces: Options For The Painting Contractor"
Joint Coatings/Forest Products Committee Charles Jourdain, California Redwood Association, Novato, CA (1-415-382-0662)

Table of Contents

• Timber Production
• Architectural Wood Products
• Finish Alternatives
• Summary
• References

Wood is a product of nature. Unlike money, wood does quite literally grow on trees. The objective of this article is to describe how wood has come to be used as a versatile and economical building material and what finishing options are available from a painting contractor's perspective that can enhance the appearance and performance of solid wood materials.

Wood develops as a functional tissue of plants rather than as a material manufactured by man for a specific end use. The process of photosynthesis, which we all studied at some time during high school biology class, is responsible for the formation of wood. During photosynthesis, carbon dioxide from the atmosphere combines with water in the leaves. This process is catalyzed by chlorophyll and energized by sunlight. Photosynthesis results in the formation of cellulose, a long chain polymer which is the main component of wood. The other major component of wood is lignin, a complex group of chemicals which bonds the cellulose molecules together in a matrix. Wood is thus a natural composite material with myriad uses, including exterior architectural elements of structures.

Wood plays a large, but frequently overlooked, role in the lives of Americans. The presence of vast forests and a readily abundant supply of wood were two of the major factors enabling the rapid settling and development of America. Wood provided fuel for cooking and heating and charcoal for industry. Wood also provided shelter and furnishings and transportation in the form of ships and wagons and ties for railroads. Today, nearly every American lives in houses built of wood and uses wood and paper products extensively in their everyday lives. Every year, each American uses the equivalent of a 100-foot tall diameter tree for wood and paper products.

Yet, due to the ability of the forest to constantly renew itself along with scientific forest management practices, the U.S. today has more trees than it had 70 years ago. In fact, one-third of the U.S. is covered with forests. This is about 75 percent of the forest that existed in 1600. Most of the reduction in forest cover was the result of urban development.

In 1989, the forest industry, state and federal governments, and individual tree farmers planted over two billion seedlings. Despite the widely held belief that forest land is being rapidly depleted, forest growth exceeds annual harvest by approximately 37 percent. Of the 731 million acres of forested land in the U.S., more than 244 million acres, or one-third of the total, are set aside by various government agencies and, therefore, are withdrawn from commercial activity.

The forest products industry in the U.S. employs about 1.3 million people and has an annual payroll of about $31 billion. This industry contributes about 4 percent of the nation's gross national product.

Not only is wood a product of nature, but it is one of the most environmentally benign materials for construction as well. Although there are obvious environmental impacts that are associated with timber harvesting, the impacts of gathering and processing alternative building materials are quite substantial. Energy requirements involved throughout the process, from raw material extraction to finished product, is on the order of 70 times higher for aluminum than for an equal weight of wood. It is 17 times higher for steel, 3.1 times higher for brick, and 3 times higher for concrete block than for wood. (Table 1)

Table 1: Energy required in the manufacture of various wall systems*

* Calculations of energy consumption include logging (or extraction), manufacture, transportation to house site, and erection.

** Committee on Renewable Resources For Industrial Materials-National Research Council. 1976. "Wood for Structural and Architectural Purposes. Wood and Fiber" 8(1):1-72

Timber Production..........(Return to Table of Contents)

Silviculture is the theory and practice of controlling forest establishment, composition and growth. Timber production is somewhat analogous to the production of crops such as corn or wheat but on a much larger time scale. Rotations for commercial softwoods may range from 25 to 80 years, depending on species of the tree and region of the country. During this time, the stand or crop is established through natural or artificial regeneration. Numerous thinnings and other cultivating practices occur, and finally the crop is harvested for timber products.

Harvesting may be done by any number of different techniques-either through selection, clear cutting, or intermediate harvest methods. The particular method used depends upon the species, site factors, and economics. Once the timber is harvested, logs are transported to mills for primary conversion. Here the logs are debarked and then sawn, edged and trimmed to rough dimensions. Rough lumber is then stacked to air dry for a period of time, or it may go directly into the dry kilns to reduce the moisture content so that the lumber will be stable. Rough dry lumber is then ready for secondary conversion where it is typically milled into final product form. At this point, the lumber is milled to particular patterns with specified dimensions and textures.

Architectural Wood Products..........(Return to Table of Contents)

The greatest amount of wood used in construction is used for framing purposes and rarely receives any type of finish. Exposed wood, used for architectural applications such as siding, trim and facia or for combined architectural and structural functions such as decking, typically receives a protective finish. Wood used in these applications is typically one of the high quality grades of naturally durable species that possess unique properties which make them suited for such uses. Solid wood siding constitutes about 12 percent of the total market for exterior siding. (Table 2)

Table 2: Exterior residential siding used in the U.S., 1987-1991*

* F. W. Dodge Residential Statistics, 1991

In 1991, well over 250 million square feet of solid wood siding was in residential construction in the United States. Additionally, several million square feet of solid wood siding are used in commercial construction each year. Premium wood siding typically is redwood, cedar or cypress. These species not only possess natural resistance to wood-destroying organisms such as fungi and termites, but they also exhibit outstanding dimensional stability. This dimensional stability means that products from these species will exhibit little shrinking and swelling during the natural changes in moisture and temperature from season to season.

The natural durability of these species results from the chemical makeup of the wood. During the process of tree growth, chemical compounds, which are toxic to fungi and insects, are deposited in the cell structure. The inherent dimensional stability of these woods is also the result of microscopic anatomical characteristics. Both durability and stability of premium wood products enhance the performance of exterior finishes applied to these substrates.

Other properties possessed by premium exterior wood products such as redwood and cedar include: excellent workability, high relative resistance to flame spread, good strength-to-weight ratio, and high insulation values. Commercial softwood species can be separated into four categories based upon their paint-holding characteristics. (Table 3)

Table 3: Characteristics of commercial softwoods for painting and finishing*

* "Exterior Wood in the South; Selection, Application and Finishes." Cassens, D.L. and W. C. Feist. USDA Forest General Technical Report. FPL-GTR-69. 1991.

Cedar, cypress, and redwood are considered the best solid wood substrates from the standpoint of exterior finishing. Pine, spruce, and fir, while sometimes used for exterior architectural siding and trim, are much more difficult substances on which to maintain finishes.

Premium exterior wood products are divided into two general grade categories based upon the presence of or lack of knots and other naturally occurring characteristics. Architectural grades are characterized by being clear, with no knots or by the presence of only an occasional small, tight knot. These grades are widely used for siding, trim and facia, but they may also be used for fine decks and other outdoor structures such as trellises and gazebos. The other general category of grades are referred to as garden or rustic grades. These grades are characterized by the presence of knots and other naturally occurring characteristics such as stain, small holes, limited warp, splits, etc.

In addition to naturally occurring characteristics, a number of manufacturing characteristics of solid wood siding can affect finish performance. These include: moisture content, grain orientation, and surface texture.

For premium architectural applications such as siding and trim, the wood should be kiln dried. Kiln drying reduces the moisture content in the wood to a range of 10 to 15 percent. This is similar to the ambient moisture content that the wood will reach in use. The results of kiln drying are greatly reduced shrinking and swelling and, thus, reduced stress placed on any surface coating. Moisture escaping wet wood will cause blistering of applied coatings. In contrast to siding, most exterior decking is not kiln dried. Penetrating water repellents and deck stains can be successfully applied to unseasoned wood as long as the surface is clean and dry.

Depending upon how lumber is sawn from a log, it will either be flat grain (growth rings parallel to the face) or vertical grain (growth rings perpendicular to the face). Vertical grain lumber is not only more dimensionally stable than flat grain lumber, but it is a better substance for protective coatings, because it does not have large exposed areas of dense latewood. Flat grain lumber is subject to grain raising, particularly if the pitch or heartwood side of the board is exposed to the weather. Grain raising can rupture brittle paint films resulting in extensive failures. Where flat grain lumber is used, the bark or sap side should be exposed, as it is much less susceptible to grain raising.

Another way to enhance the performance of exterior finishes is to specify a saw-texture rather than a smooth surface. Saw-texture surfaces will absorb much more finish, hold it better, and show fewer minor surface irregularities. As another precaution against grain raising, saw-texture surfaces should be specified when using flat grain lumber. Smooth planed surfaces may experience burnishing as a result of the matching process. This burnishing, or mill glaze, can reduce the penetration and adhesion of applied finishes.

In addition to the various grades available, siding, trim, facia and decking are produced in a number of different patterns and dimensions. Siding patterns are available in bevel, rabbeted bevel, tongue and groove, shiplap, and board and batten. Siding is sawn in dimensions from 4 to 12 inches in width. Each of these patterns may be available in different thicknesses and with various details such as V grooves and channels. Decking is typically surfaced-four sides of radius edged, and is available in sizes from 1x4 inch, through the popular 5/4x6 inch, and to the more traditional 2x6 inch.

Finish Alternatives..........(Return to Table of Contents)

Siding and decking receive the vast majority of all architectural finishes applied to exterior solid wood products. Despite the recommendations of siding and decking manufacturers, that their products be protected by finish systems, some consumers will still leave wood siding and decking unfinished. If the consumer is aware of the natural weathering process of wood and is willing to accept a possibly shorter service of life, then the simplest finish alternative may be no finish at all.

Usually, however, the case is to maintain or enhance both the appearance and the long-term service of siding and decking. To do this, a number of finish alternatives -- each with its own level of protection, aesthetics and maintenance work load are available. These range from the simplest clear water-repellent solutions through multi-coat paint systems.

Clear water repellent preservative solutions are often thought of as treatments rather than as finishes. These products offer relatively short-lived water repellency through the incorporation of paraffin wax, silicates or various oils. Imparting water repellency to the wood reduces stresses caused by rapid absorption and desorption of liquid water. These stresses result in surface checking, splitting, and dimensional instability in the wood. The term "sealer" is sometimes inappropriately used to refer to water repellents. Water repellents allow water vapor to pass in and out of the wood and, thus, in a sense, do not seal the surface. Water repellent products used on exterior wood should, at the very least, contain a mildewcide to prevent the growth of mildew on the wood surface. Water repellent preservative solutions typically have a much higher concentration of fungicide for use on products located near the soil or in other decay hazard situations. Numerous water-repellent products on the market today also include some form of ultraviolet (UV) light protection to help maintain or stabilize the color of the wood surface.

Air quality concerns, in the form of restrictions on Volatile Organic Compound (VOC) levels in architectural finishes, have resulted in many newly formulated products in this and other finish categories. Common formulations may be 1) high-solids solvent-borne, 2) exempt solvent-borne, 3) water-based acrylics, 4) water-dispersible oil solutions. In any case, these "natural finish" water repellents are at least durable of the available architectural finishes for solid wood products, and they may require re-application within one or two years.

Transparent stains are a relatively new product in this field. They occupy a niche between the clear water repellent and the more traditional semi-transparent stains. Transparent stains are slightly tinted as a result of the incorporation of transparent iron oxide pigments used to reflect UV radiation and, thereby, to maintain the beauty of the new wood. The colors available are typically limited to a handful of natural cedar and redwood tones. These products also incorporate water repellents and mildewcides. Refinishing cycles depend on the severity of exposure, but typically they are two to three years.

Semi-transparent stains are probably the most popular and widely used of all the natural finishes. These products are available in a wide variety of colors but, like the other natural finishes, they are penetrating and, thus, do not fail by blistering, flaking, or peeling in the way that film-forming coatings may fail. When used for decks or other walking surfaces, semi-transparents should be specifically formulated for such applications. Otherwise, these products may deteriorate rapidly under foot traffic. Semi-transparent stains usually require re-application in two to five years.

Other natural finishes worth mentioning are the bleaching or weathering stains. These products are applied much like semi-transparent but they incorporate chemical bleaches, which are activated by moisture. This results in the wood's surface taking on a bleached, driftwood gray appearance that is highly desirable by consumers, particularly those in coastal regions. In some respects, this bleaching mimics the natural graying or weathering of wood left unfinished over time, but accomplishes this on a much more accelerated and uniform basis. These bleaching stains should be periodically followed by the application of clear water repellent solutions.

Solid body, solid color, or solid hide stains are much more similar in nature to paint systems than they are to semi-transparent stains. These products, whether water-based acrylics or oiled-based solvent-borne systems, form a coating on the wood surface, hiding most of the wood and usually dramatically altering the color of the natural grain and character of the wood. When applied over extractive rich woods such as redwood and cedar, acrylic solid color stains should be applied over a top quality oil- or alkyd-based primer specifically formulated for extractive rich woods. As with the other finishes previously discussed, where the manufacturers' formulations allow, a minimum of two coats should be applied for the best performance. Solid body stains may need re-application every five to seven years. Coatings such as these are not recommended for decks and walking surfaces.

Varnishes and other clear film-forming finishes are generally not recommended for use on exterior wood due to their rapid failure from UV light. There is a film-forming, translucent finish, which has been specifically developed for such use and is recommended by many manufacturers. This European product is a multiple coat system which is relatively expensive, has stringent application instructions, and provides a uniquely attractive and durable surface. Re-application of a topcoat is recommended every three years. This product will fail by peeling if it is not maintained properly, resulting in extensive surface preparation efforts prior to recoating.

The ultimate in terms of long-term protection and extended maintenance cycles for solid wood siding is a high quality paint system. A three-coat paint system, one prime coat and two topcoats, properly applied to a well-prepared surface, may last ten or more years. An oil- or alkyd based primer is recommended for extractive rich wood such as redwood and cedar. As an alternative, water borne acrylic or water dispersible oil "stain blocking" primers may be used on such woods. Two coats of such a primer, with proper drying time between applications, may be required to provide stain-blocking effectiveness. Even in the case of oil-or alkyd-based primers, the recommended application rate must be achieved for optimum performance. It is also highly recommended that the prime coat be brush applied. Back brushing or back-rolling of a spray-applied primer provides shearing action required for proper surface penetration and adhesion. High quality acrylic latex top coats, preferably 100 percent acrylic latex, will provide the greatest level of durability and color retention. Regardless of the finish alternative selected by your customer, backpriming or prefinishing of the product on all surfaces is highly recommended. Backpriming not only enhances the stability of the product, but it can help reduce extractive staining problems as well.

A recent trend toward factory prepriming and prefinishing offers significant advantages over field applied finishes. These advantages include controlled temperature and humidity conditions, easily monitored application rate, and proper storage and handling of finished materials.

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

Of the multitude of building materials used in residential and non-residential construction, few can match the natural beauty of solid wood products.

The beauty of wood goes well beyond its appearance and performance. Wood is one of, if not the most, environmentally sound of all building materials. It is renewable and easily recycled, and it is energy efficient in terms of both production and utilization.

Responsible, scientifically sound forest management practices are ensuring a continued supply of high quality wood products to the marketplace.

The appearance and long term durability of exterior wood products such as siding and decking can be significantly enhanced by the proper application and maintenance of architectural finishes.

These finish systems range from the natural clear water repellents which require frequent re-application and provide some protection from the weathering process through the multi-coat paint systems which cover the surfaces of the wood completely but provide a high level of protection and the greatest interval between finish application.

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

1. Forest Statistics of the U.S., 1987. USDA Forest Service, pp 10-11, Table 1.

2. 1988 Annual Survey of Manufacturers, Bureau of the Census, U.S. Department of Commerce.

3. Boyer, J. 1991. "Alternatives to Forest Harvest and Wood Use: The Environmental Impacts are Substantial." Wood Product Demand and the Environment. Proceedings. Forest Products Research Society. Nov. 13-15, 1991.

4. Steida, C. 1991. "Environmental Impact of Using Wood as a Construction Material." Wood Product Demand and the Environment. Proceedings. Forest Products Research Society. Nov. 13-15, 1991.


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.© 1994-2009 JohnC.Leeke