John Leeke's Historic HomeWorks™

    207 773-2306      26 Higgins St. Portland, ME  04103
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"Factory Finishing Of Exterior Wood Products"
Joint Coatings/Forest Products Committee Chairman: Charles Jourdain, California Redwood Association, Novato, CA (1-415-382-0662) Steve Bussjaeger, HIS Paint Manufacturing Company, Oklahoma City, OK John Cooper, Duckback Products Inc., Colorado Springs, CO Tom Daniels, Cabot, Newburyport, MA Keith Kersell, The Pacific Lumber Company, Scotia, CA Arnie Nebelsick, Western Red Cedar Lumber Association, Vancouver, BC Robert Springate, Tru-Test Manufacturing Co., Chicago, IL Sam Williams, USDA Forest Products Laboratory, Madison, WI

Table of Contents


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

This article contains an overview of factory finishing for wood substrates. The advantages of factory finishing over field applied finishes, the types of equipment used, the types of finish, and suggestions for installation and maintenance are included.

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

Factory finishing exterior wood products prior to installation is recommended by many research groups, paint companies, and wood products associations. These include USDA Forest Products Laboratory, Western Red Cedar Lumber Association, California Redwood Association and Western Wood Products Association. Installation of factory-finished lumber is one of the best ways to achieve excellent performance from wood and wood-based products used outdoors. Quality control problems, which may occur from field application of finishes, can usually be avoided. These include adverse weather conditions, surface soiling, improper surface preparation, high moisture content, insufficient film-build, lack of quality application, and weathering of wood prior to finish application. For example, weathering of smooth-planed wood for as little as two weeks can damage the surface sufficiently to cause early failure of subsequently applied paint.

This is not to imply that good performance of painted wood cannot be achieved through field application of finishes. Finishes have been field applied to wood for centuries and have usually given excellent performance. However, recent changes in the wood and paint industries and in construction practices make it more difficult to obtain maximum performance from these products when field applied. For example, the switch from solvent-based oil-borne formulations to water-based latex formulations requires greater attention to ambient temperature during and following application. Factory finishing gives greater control of application conditions, such as temperature.

Factory finishing usually provides cost savings to builders and consumers, additional value-added product sales opportunity to lumber suppliers, and extended warranties to consumers. Builders benefit because the additional cost of the prefinished lumber is usually considerably less than the cost of field application of finishes, its use eliminates weather caused delays associated with field application, and it makes possible more efficient subcontractor scheduling (Table 1). Prefinished lumber can be packaged and delivered when the builder is ready for the material. There may also be less on-site waste. Lumber suppliers may benefit by having an additional product line, one with added value over unfinished lumber. Many factory-finish companies offer long-term warranties, some as long as 15 years. Depending on the manufacturer, the warrantee may guarantee against cracking, peeling, blistering, erosion, chalking and yellowing.

Factory finishing is a versatile process that can be used on a wide range of building materials, such as siding, decking, trim lumber, fencing, and joinery (doors and windows). These materials are often available in a wide variety of wood species, surface types, and wood-based composites such as hardboard, plywood, and particleboard. The color selection is often much greater than available from other prefinished wood substitutes, such as vinyl and aluminum siding. These substrate and color options give the consumer a wide range of choices. Factory finishing can include priming only, a water repellent (WR) or water-repellent preservative (WRP) pretreatment, or a full finish system. Most importantly, factory finishing can take advantage of specialized finishes, applied under optimal conditions at the manufacturer's recommended spread-rates. This should result in maximum performance in terms of substrate protection and finish system service life.

Durability, Maintenance, and Service Life..........(Return to Table of Contents)

Manufacturers of coatings specifically formulated for factory finishing often work closely with lumber suppliers and prefinish companies to assure the best match of finish with substrate. This attention to the specific product needs should translate to maximum durability and protection against nature's harmful elements. The wood resource available for construction is changing. (1) The coatings industry is also changing. The Clean Air Acts of 1990, requires coatings to have decreased levels of volatile organic compounds (VOCs). Many of the water-based coatings available for exterior use, are not compatible with cold weather field application. It is essential that the prefinish company work with the finish and lumber suppliers to assure compatibility of materials. The wood species available for siding are changing. For example, vertical-grained western red cedar and redwood were traditionally used for siding. Many other species are now available (western hemlock, eastern white pine, yellow poplar, Douglas-fir, and southern pine). These siding materials are usually available only in flat-grained orientation. Flat-grained wood is, in general, more difficult to finish than vertical-grained wood.

Factory finishers may use large volumes of wood and usually have the ability to work with lumber suppliers to assure that grade standards are met, that the wood meets moisture content (MC) specification, and to achieve economy-of-scale savings through large volume purchases. They usually work with paint suppliers to assure that the finish will meet the substrate requirements. The finish formulation may have to be modified for different substrates and is often specifically designed to meet substrate requirements.

Many of the wood species entering the market do not have the durability (i.e. resistance to decay) that is usually found with redwood and cedar. Factory finishing offers the ability to pre-treat the lumber with a WRP to give the lumber added durability. The lumber is usually primed on all sides, which decreases problems with extractives bleeding. Since the whole surface of siding is finished, there will likely be no gaps in the finish if the wood shrinks in service. Application of the finish under a controlled climate should insure the ideal environment for coating application, drying, and curing thus assuring maximum service life for the finish. The net result should be a superior long-lasting finished wood product that minimizes future maintenance.

Benefits of Factory Finishing..........(Return to Table of Contents)


Substrates For Factory Finishing..........(Return to Table of Contents)

Painted products have had widespread acceptance by architects, builders, painting contractors, interior designers, and consumers for well over 100 years in the United States, western Europe, and other parts of the world. Factory finishing should be equally compatible with almost any lumber product that needs finishing.

Wood Products that can be Factory Finished:


Application Equipment..........(Return to Table of Contents)

The two most common application methods for factory applied coatings to wood substrates are a flow coater and vacuum coater. Roll coaters are also used, but to a lesser degree and when used, they give uniform application to specific substrates using specialized, multi-application coatings.

Flow Coater
Flow Coater Flow coaters are currently the "work horse" of the factory finishing industry and are the most common equipment used for application of factory-applied coatings (Figure 1). Flow coaters can be configured to apply the finish to the top, sides and/or bottom of the substrate. Substrates can include wood, wood based composites such as fiberboard, or oriented strand board (OSB) or other manufactured siding materials such as cement/wood fiber composites. Once the finish is flooded onto the substrate, a set of knurled driver rollers pull the wood substrate through the machine, while insuring that the finish is evenly applied. A series of high-speed brushes then works the finish into the substrate. Some flow coaters are equipped with a pneumatic "air knife". This air knife can be used in place of the high-speed brushes or in conjunction with the brushes. Air knives are more commonly used when smooth planed wood substrates are being coated because the air knife can leave a smoother finish. High-speed brushes are more commonly used on rough textured surfaces. The brushes and/or air knives can be set to assure that the film-build of the coating is consistent and meets specifications.

Advantages of a Flow Coater:

Vacuum Coater
Vacuum coaters use a vacuum to apply the finish to all sides of wood or wood composite substrates (Figure 2). The vacuum serves two purposes. First, it draws the finish from the bottom of the application chamber into the application area, atomizing it, much like the action of a tornado. Second, it creates a flow of air through the entry and exit templates, removing excess finish and smoothing the finish surface. The contour of the in-feed and out-feed template located on the chamber are cut to the contour of the substrate having approximately 3 mm (1/8 inch) of space for air to pass over the substrate. When the coated substrate exits the application chamber, the air passing around the substrate and through the template, draw the excess finish back into the application chamber. As with the flow coater, the air flow and template tolerances can be set to assure consistent film-build.

Advantages of a Vacuum Coater:


Driers/Air Drying..........(Return to Table of Contents)

In field application of finishes to wood, the coating forms by air circulation over the surface of the finish. Oil-based finishes form a film through a process of solvent evaporation and oxidation of the oil or similar binder. Water-based finishes form a film through water evaporation and a process called coalescence. Temperature, humidity, air circulation, and coating formulation all affect the time necessary for a finish to form a film. Because factory finishes are applied in a controlled environment, air drying can be complemented or replaced by driers. These driers can improve the efficiency of factory finishing by increasing the rate of drying, thus eliminating the need for drying racks and decreasing the space requirements in the factory. The most common processes used for increasing the drying rate involve high-temperature forced-air drying, infrared radiation, or a combination of both. By using finishes specifically formulated for use with these drying systems, consistent quality products can be produced, often increasing the plant throughput over that possible using air drying.

In forced-air drying, the coated wood is passed through an oven. Forced-air drying is similar to conventional air drying in that the finish dries from the surface down. By carefully controlling the humidity, temperature, and air circulation of the oven to insure that the finish dries throughout without premature hardening the coating surface, the drying rate can be greatly increased over that obtained through racking and air drying. Primers and topcoats can be applied and cured rapidly to give a consistent quality product. Infrared (IR) dryers use IR radiation to cure the finish. The IR radiation heats the film much like a microwave oven. The energy in the IR radiation is absorbed throughout the coating, drying and curing the coating from the inside out. As with forced air drying, the process parameters can be carefully controlled to assure a consistent quality product.

Driers/Air Drying..........(Return to Table of Contents)

While many different types of finish may be factory applied, the predominant factory applied product today is factory-primed siding that is intended to have a paint or solid-color stain applied in the field. Factory application can assure better air quality through solvent recovery or incineration of VOCs, particularly during application of solvent-borne finishes such as alkyd primers. Alkyd-based primers or specially formulated stain-blocking acrylic primers are recommended for use on western red cedar, redwood and other extractive-rich substrates. Acrylic primers are usually used on non-extractive-prone surfaces such as hardboard, pine, cypress and spruce.

Specifically formulated for machine finishing process, acrylic primers and acrylic top-coats have superior block resistance over traditional latex paints and stains. Blocking is the undesirable sticking together of two painted surfaces when pressed together under normal conditions or under specified conditions of temperature, pressure and relative humidity. Block resistance is evaluated using ASTM test method D2793-93a "Block Resistance of Organic Coatings on Wood Panel Substrates". (2) Factory-applied acrylic finishes are formulated to pass a Class D test, however, we recommend that slip sheeting material be placed between boards to prevent blocking in case the coated lumber is subjected to more severe conditions. The industry has traditionally used slip sheet material consisting of either a polyethylene film, or a wax coated or impregnated paper. Coatings manufacturers are currently conducting extensive research and development to further improve the block resistance of the acrylic finishes. Acrylic primers that do not require slip sheeting are now available.

Primers
Both oil and acrylic high-hiding primers are available from most finish manufacturers and are usually formulated to prevent extractives bleeding and dry fast.

Acrylic Paints
Acrylic latex topcoats are tough, abrasion- and fade-resistant finishes that are exceptionally long lasting. They are typically available in a wide range of colors and are formulated specifically for machine application. They often include mildewcides and ultraviolet (UV) stabilizers.

Solid Color Stains
Solid Color Stains Solid color acrylic or oil-based stains may be formulated with a variety of resins or combinations of resins and are useful for finishing siding, deck furniture, and fencing. They are opaque, mildew and fade-resistant, and sometimes contain a WR. To get adequate performance, they should be used over a primer. When finished with adequate film-build, they generally give good service life.

Semi-Transparent Stains
Semi-transparent stains enhance natural wood grain by blending deep-penetrating oil with low-hiding pigments. They may include a WR and mildewcide and since they do not form a film, they are resistant to cracking, blistering, and peeling.

Oil-Based Decking Stains
Oil-based decking stains are usually formulated with alkyd resin and often contain a WR and mildewcide. They can be used for all types of wood decking and fencing and provide a tough, scuff-resistant finish. They penetrate well into the wood to give a more natural finish than the opaque film-forming finishes.

Transparent Stains
A number of finish suppliers have transparent stain formulations that contain additives to protect the finish and substrate from UV radiation. They are often available in a variety of tones and may include UV radiation stabilizers such as transoxide pigments, ultra-fine titanium dioxide, and/or hindered amine light stabilizers (HALS) to give them slightly better weathering performance than the clear WRs and WRPs.

Clear Water Repellents
Oil-based WRs and WRPs are usually formulated with a blend of resin and oil to penetrate into wood and provide superior protection from water and weathering. They generally do not give long service life compared to pigmented finishes, but they give excellent protection to wood when used as a pretreatment. Some are lightly tinted and these give slightly better service life than the clear finishes. With the performance limitations of these finishes, it is probably not worth the expense to factory finish, except as a pretreatment for a full primer and paint system. In this use, they are extremely beneficial to both improve wood durability and to increase finish service life and it is probably well worth the extra step.

Some finish suppliers are formulating water-based clear WRs and WRPs with a water-reducible oil and acrylic resins. These products allow the natural color of the wood to show, but as with the oil-based clear WRs, their performance is much diminished compared with the highly pigmented acrylic paints described above.

Handling, Installation & Field Finishing..........(Return to Table of Contents)

Moisture
Premature failure of coatings on wood substrates is often a sign that excessive moisture has absorbed into wood products. It is important that the wood products such as siding and sheathing maintain a moisture content below 15%. There are four primary sources of moisture that can affect wood and the coatings on them: precipitation from rain, snow, and dew, roof or plumbing leaks, ground moisture, and condensation of moisture-laden air flowing out of a structure through the walls and around joinery. Most moisture-related problems are preventable through proper design and construction techniques. To control moisture and prevent problems, take the following steps: 1) ventilate bathrooms and laundries to control indoor relative humidity; 2) use vapor retarders and air barriers; 3) ventilate attics and dehumidify damp basements; and 4) install flashing especially around windows and doors and at wall and roof intersections.

Storage
Until installed, factory-finished lumber should be protected from sunlight, moisture, dirt and other contamination (Table 2). Lumber should be stored in a dry location. If it must be stored outside, place a plastic cover over soil to minimize moisture absorption from the earth. Then place lumber off the ground on blocks that provide level support. Finally, protect the stack of lumber with a waterproof cover. Do not completely seal the bundle, however, as good air circulation is necessary to bring the wood to equilibrium moisture content with its surroundings.

Lumber Installation
Avoid marring and scuffing the factory-finished lumber during cutting and handling. Saw textured or resawn lumber will take more stain/paint, which will result in a longer lasting finish. Back cutting can reduce splintered or frayed butt ends. Exposed butt ends resulting from field-cuts must be coated to prevent water migration, which may cause extractive bleeding and peeling. All siding should be installed according to industry standards over 15 lb. asphalt felt or housewrap. It also helps the siding performance, in some climates, to install it over furring strips to give an air gap between the siding and the felt or housewrap. If the siding is furred out in this manner, be sure to screen the top and bottom to eliminate entry of insects into the air space.

It may sometimes be necessary to field coat factory-finished siding if the on-site storage and handling was poor resulting in dirty siding, nails holes, frayed or damaged cut ends of siding, and hand prints. This is more problematic with siding installed smooth face out. For field applications of topcoats on pre-primed lumber or the application of an additional top-coat, use a finish that is approved for use by the factory finisher.

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

Pre-Primed Lumber
Factory primed or pre-primed lumber (i.e. those products having only a primer) do not offer much protection from the weather. They must be protected with a field-applied topcoat as soon as possible after installation and therefore should not be installed during adverse weather condition. The adhesion of the field applied top-coats may be compromised if they are exposed to the weather. In addition, the primers do not give very good protection against the absorption of water. Unless a longer period is specified by the factory finisher, the pre-primed lumber should be top-coated within 14 days of installation. Failure to apply the topcoat to the primer in a timely manner can result in chalking of the primer and checking and raised grain of the substrate. If the siding is not finished within the time specified by the factory finisher, it may be necessary to clean the surface and reprime it.

Stain/paint can be brushed, rolled or sprayed. Before applying topcoat, it is best to have the moisture content of the wood siding and sheathing below 15%, particularly with oil or oil-alkyd finishes. Brush application insures best coverage and protection. For best results, apply two coats of 100% acrylic paint or stain. Quality oil-based paint and solid color stain may be used. However, two coats of 100% acrylic paint/stain will generally outlast oil-based coatings. Apply stain/paint according to the manufacturer's recommended spread rates and other specifications.

By following proper lumber storage and installation instructions, factory-finished lumber should be ready to install when delivered. Ensure that all "slip sheeting" stays between boards until it is installed. Treat the factory-finished lumber as your would any other high value building material; store it inside protected from moisture and physical damage. During installation, touch up cut ends and any areas that may have been damaged during storage or installation with finish provided by the factory finisher. For some colors, fasteners are available that match the color of factory finished lumber (Figure 3). Some manufactures may extend their warrantee if additional coats of their finish are field applied. As with pre-primed lumber, additional field-applied top-coats may be brushed, rolled or sprayed. Brush application or back brushing insures best coverage and protection. Apply stain/paint according to the manufacturer's specifications including spread rates.

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

1. Jourdain, C., J. Dwyer, K. Kersell, D. Mall, K. McClelland, R. Springate, and S. Williams. Changing Nature of Wood Products-What Does it Mean for Coatings and Finish Performance? J. Coatings Tech. 71(890):61-66, 1999.

2. Annual Book of ASTM Standards, Standard Test Method for Block Resistance of Organic Coatings on Wood Substrates. D 2793-93 (Reapproved 1987), Vol 06.01, p.402-403, 1991.

Figures

Figure 1. Picture of a flow coater.

Figure 2. Picture of a vacuum coater.

Figure 3. Closeup of factory-finished siding and color-matched fastener.

Tables

Table 1. Features, advantages, and benefits of factory finishing.

FEATURES
ADVANTAGES
BENEFITS

Machine Finished
Factory Controlled

Consistent availability


Uniform application color consistency


All siding pre-finished at time of installation


Table 2. Installation and finishing requirements
Storage


Installation

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