John Leeke's Historic HomeWorks™
	207 773-2306      26 Higgins St. Portland, ME  04103
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"Surface Preparation and Refinishing of Wood"
Joint Coatings/Forest Products Committee Chairman: Jeff Fantozzi, Pacific Lumber Inspection Bureau, Bellevue, WA Steve Bussjaeger, H-I-S Manufacturing Co., Oklahoma City, OK Richard Carlson, APA-The Engineered Wood Association, Tacoma, WA Vern Donegan, ICI Paints, Strongsville, OH Bill Feist, USDA Forest Products Lab, Madison, WI Louis Wagner, American Hardboard Association, Palatine, IL

Table of Contents

• Introduction
• Wood Products Used Outdoors and Typical Applicatioon
• Finishing Characteristics of Wood and Wood-Based Products
• Surface Conditions and Preparation
• Refinishing
• Removal of Finishes
• Special Techniques for Professional Finish Removal
• Removal of Lead-Based Paints
• Finish Selection and Performance Expectations
• Special Refinishing Needs
• Conclusion/Summary

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

The abundance and versatility of wood have generated the extensive use of wood products in North America. These products come in many forms, whether solid-sawn lumber and siding or panel products such as plywood, hardboard or Oriented Strand Board. The unique characteristics of these wood products make them suitable for many applications. For the most part, knowledge about these characteristics has been developed through practical experience rather than through scientific investigation.

Wood products are often used outdoors in an environment that places many demands both on the wood substrate and on the finish. Just as there are many types of wood products to choose from for outdoor applications, there are a variety of finishes that can be applied. These finishes include clear finishes, which reveal and accentuate the natural beauty of wood; semi-transparent stains, which impart a rustic appearance; and paint or solid-color stains, which can be obtained in a multitude of colors. Not all finishes are appropriate for all exterior wood products.

This paper describes the refinishing of wood products used outdoors, including proper application of finishes to solid and panel products. Methods for selecting and applying various exterior wood finishes are presented. Finally, the failure and discoloration of wood finishes are discussed and methods are given for preventing these problems.

Wood Products Used Outdoors and Typical Applications..........Table of Contents

There are a variety of outdoor applications where wood products are commonly used. The application, as well as the unique characteristics of the wood product used, will affect the durability and choice of any finish applied to it. In addition, treatment with wood preservatives or fire-retardant chemicals also affects the finishing and refinishing characteristics of the product. Table 1 provides common wood products and their applications.

Table 1.

1) Recommend overlaid plywood
2) Consult the manufacturer

Finishing Characteristics of Wood and Wood-Based Products..........(Return to Table of Contents)

Solid-Sawn Wood
The refinishing of wood with film-forming finishes like paints or solid-color stains is strongly influenced by the original paintablility. The paintablility of various softwoods and hardwoods used in the United States is described in Table 2 in terms of the natural wood characteristics of density, presence of latewood, texture and manufacturing characteristics such as ring orientation. This table also provides information on the relative weathering characteristics of the species.

Softwoods -- Redwood and cedar are low density woods and old growth material typically has narrow bands of latewood, whereas southern yellow pine and Douglas-fir are higher in density and may have wider bands of latewood. The higher density latewood does not accept a finish as well as lower density wood. Growth rate will also affect the performance of some finishes. Faster growing trees have wider growth rings with larger bands of latewood. In general, regardless of the species, film-forming finishes perform better on material with narrower growth rings.

Table 2. Finishing Characteristics of Various Species-*

*- Includes wood and plywood. Woods ranked in category V for ease of keeping well painted are hardwoods with large pores that need filling with wood filler for durable painting.
**- Values are based on close grain, vertical grain surfaces. All other values are for flat grain surfaces.
***- Plywood, lumber, oriented strandboard and hardboard with overlay or low-density surface.
(--) Indicates no information available.

Table adapted from a similar table in the USDA Forest Products Laboratory Wood Handbook, 1987, Madison, WI

Surface roughness also plays a role in determining finish performance. Rough or saw-textured wood has more surface area than smooth planed wood and, consequently, requires more finish to achieve the proper coverage. As a result, finishes tend to last longer, sometimes twice as long, on rough surfaces than on smooth surfaces. In addition, smooth surfaces of some species like western red cedar occasionally exhibit a phenomenon known as "mill glaze". Not well understood, mill glaze is created during the planing or drying process; the end result being a smooth surface that does not accept certain finishes, like the direct application of solid body stains on smooth cedar. For this reason, some trade associations and manufacturers do not recommend solid body stains on smooth cedar siding in certain parts of the country.

Hardwoods
The best hardwoods for painting are fine, uniform-textured (small-pored) woods with medium to low density such as yellow-poplar, magnolia, and cottonwood. These category III hardwoods should perform as well as softwoods in category III and IV because the hardwoods have less tendency to split and do not have distinct latewood bands. The category IV hardwoods can be painted using standard procedures, however, repainting may be difficult unless all old paint is removed first. On some domestic hardwoods, paint tends to scale off in rather large flakes, apparently regardless of the grain of the wood beneath the paint. The pores of the category V hardwoods are so large that they are not filled and leveled off properly by ordinary house paint. The pores consequently become the foci for early paint failure. Therefore, the pores must be filled with exterior grade wood filler paste prior to painting.

Preservative Treatment
Preservative-treated lumber is often used in decks and fences. The most common type of preservative is Chromated Copper Arsenate (CCA), however, Ammoniacal Copper Zinc Arsenate (ACZA) and Ammoniacal Copper Quaternary (ACQ) are becoming more popular. Properly applied, preservatives of this type will not leach out and water repellents, stains or paints can be successfully applied over them. Because treated lumber is often not dried after treatment, penetration of a finish can be reduced thus it is recommended that the wood be allowed to dry prior to finishing.

Plywood
Plywood has the advantage of alternate layers of veneers laid with the grain direction perpendicular to each other which gives dimensional stability and prevents cupping. It is most commonly manufactured in four-by-eight foot sheets but may also be cut in strips to produce lap siding. The surface may be either smooth or rough, or it may be overlaid with a medium-density resin impregnated paper. Panel siding is often grooved to a depth of about one-quarter inch to give the appearance of vertical boards.

Certain grades of plywood siding may be finished with semi-transparent penetrating stains. All grades of plywood siding can be finished with solid- color stains or acrylic latex paints. Exterior sanded plywood may be used in various outdoor applications such as soffits, but it is not recommended for use as siding. Only acrylic latex paints are recommended for use on sanded plywood. Paper overlaid plywood provides an excellent surface for all types of paint that are formulated for use on wood-based products. Some manufacturers of overlaid plywood only recommend top quality acrylic latex formulations if solid-color stains are used.

Hardboard
Hardboard is commonly applied as horizontal lap siding in varying widths or as four foot wide panels. This material is formed from wood fibers that are compressed under heat and pressure to form panels with a hard, smooth surface that is an excellent base for paint. In some cases a wood grain pattern is formed on the surface to produce a slight texture.

All hardboard produced as siding is either prefinished at the factory or factory prime coated. It is critical that the edges cut on site be primed before lengthy exposure to weather because water is absorbed through the edges and can cause edge thickness swelling. Also, an adequate thickness of topcoats is required on the edges. Factory primed hardboard siding should be finished within 60 days following installation. Hardboard must be finished with paint rather than a stain or other natural finish. Consult both the siding and paint manufacturers.

Overlaid Oriented Strand Board (OSB)
OSB is manufactured from flat strands of wood which are glued together under heat and pressure. Strands in the faces of panels are oriented with fibers parallel to the long direction of panels. Strands in the core of panels are oriented perpendicular to the face strands so the material is essentially layered similar to plywood. Also, like plywood, it has good dimensional stability and freedom from cupping. OSB siding is available in four-foot wide panels or as lap siding in 16-foot lengths and in various widths. OSB siding is embossed with a wood grain pattern although it is also available with a smooth surface. The face of OSB siding is overlaid with resin-impregnated paper or a fiber-based overlay which provide a surface with excellent finishability. The face surfaces and all edges of OSB siding are pre-primed at the factory. OSB siding manufacturers generally recommend two coats of top-quality acrylic latex paint. Specific recommendations vary, however, so check with the siding manufacturer when selecting a coating.

Wood Composites
Composite sidings are manufactured from layers of veneer and reconstituted wood fiber. Some composite panels consist of veneer face and back surfaces and OSB core. Naturally occurring voids, such as knotholes in the outer veneers, are filled with OSB material during the pressing operation. Filled knotholes resemble a solid knot but hold a finish much better. Composite panels manufactured from veneer and OSB are available as panels, lap siding or in V-groove patterns. Other composite panels are similar to plywood but have an embossed hardboard face.

Solid-color stains and acrylic latex paints may be used on composite panels. However, recommendations vary according to surface characteristics and the siding face grade. Check with the siding and paint manufacturers for complete recommendations.

Surface Conditions and Preparation..........(Return to Table of Contents)

Wood surfaces that are exposed to the weather need to be protected from the elements of ultraviolet radiation, water, mold and mildew if they are to perform satisfactorily and continue to be aesthetically pleasing. Cleaning with a stiff bristle brush or power washing are two methods commonly used to prepare surfaces for refinishing. For comments on power washing, see the section entitled "Special Techniques for Professional Finish Removal".

Note: The use of non-metallic, stiff bristle brushes is often recommended in the following sections. Never use steel wool or wire brushes to clean wood surfaces because small iron deposits may be left behind. These deposits can react with certain water-soluble extractives in wood like western red cedar, redwood, Douglas-fir and oak to produce dark blue-black stains on the surface. In addition, additives present in some semi-transparent penetrating stains and water-repellent preservatives may cause surface iron to corrode. The corrosion products may then react with certain wood extractives to form a blue-black, unsightly discoloration that becomes sealed beneath the new finish system. Metal brushes can also damage the surfaces of plywood and overlaid products.

Dirt
An automobile that is kept outside will require periodic cleaning if it is to maintain its appearance. Wood that is left exposed to the weather, be it the siding on a house, a deck or fence or outdoor furniture, will collect dirt and requires periodic cleaning. Dirt will accumulate on any surface regardless of the finish used, but areas that are protected from sun and rain such as porches and sidewalls with wide roof overhangs tend to collect dirt and water-soluble materials that interfere with adhesion of the new coatings. Salts can build up under overhangs and should be removed prior to refinishing. Dirt and salts can be removed by scrubbing with a non-metallic bristle brush and water. Stubborn stains can usually be removed by scrubbing with a commercial cleaner or detergent followed by thorough rinsing. If the discoloration persists, it is probably not dirt.

Mildew and Molds
Mildews and molds produce a fuzzy surface growth of fungal spores and may range in color from yellow to black. These organisms feed off of sugars and starches readily available in the wood or other substances that have collected on the painted surface. While mold and mildew do not affect the strength of the wood, their presence can be unsightly. Mold and mildew can be removed from a surface by cleaning with a solution of bleach and water. Depending on the severity of the condition, anywhere from one to two quarts of bleach per gallon of water is appropriate. Pre-mixed commercially formulated bleaches are also available. Wear rubber gloves and protect eyes when working with bleach solutions. Thorough rinsing of the wood and any surrounding surfaces and vegetation is required after cleaning with bleaches.

Caution: Household bleach should never be mixed with detergent containing ammonia. Fumes can be fatal.

Chalking
Chalking can occur on surfaces that have been painted or stained. Chalking results when a paint or stain film gradually weathers or deteriorates, releasing the individual particles of pigment. These act like a fine powder on the surface and can usually be rubbed off with the hand. Chalking should be removed prior to refinishing or when it begins to discolor other surfaces or fades. The old surface should be scrubbed thoroughly with a detergent solution to remove all old deposits and dirt. The surface should then be rinsed thoroughly with clean water before refinishing.

Figure 1. Washing surfaces with soap and water removes dirt and chalk.

Flaking or Peeling
Film-forming finishes, such as paints and solid-color stains, can fail by flaking or peeling if not maintained properly. These symptoms can be caused by exposing the wood to the weather for too long a period of time prior to finishing. It can also result from inadequate cleaning of the weathered finish prior to refinishing. With paints and stains that use primers, inter-coat peeling and flaking can also result from allowing too much time between the primer coat and topcoat in a new paint or stain job. Generally, no more than two weeks should pass before the topcoat is applied. Otherwise, follow the manufacturer's recommendation. Loose finish due to flaking or peeling should be removed by first scrubbing or washing the surface then scraping or brushing the surface with a stiff bristle brush.

Figure 2. Removing loose, flaking or peeling paint with a scrapper.

On smooth surfaces, the edges of any remaining coating should be sanded to "feather" them into the bare wood. Ignoring this step can result in premature failure of the subsequent finish for a number of reasons. The abrupt edge between the old film and the bare wood creates a weak point in the new coating. In addition, the bare wood where the old paint has failed may be degraded to the point where a good bond with the new paint can not be achieved. Sanding the edges and the exposed wood surface not only smooths the edges of the old paint but also provides a sound surface for future paint adhesion. Figures 3(a) through 3(f) illustrate a typical scenario. Figures 3(a) and 3(b) show a paint system that has been scraped to remove loose paint. Following repainting, note that the new paint film is extremely thin at the place where the film gaps the previously painted and unpainted portions (Figure 3(c)). The new paint will show an abrupt surface discontinuity and this is a weak point in the new coating. Within a short time, the new film will crack at this thin point, water will flow to the paint/wood interface, and the paint will fail again (Figure 3(d)). In contrast, Figure 3(e) shows a feathered edge and the weathered surface of the wood sanded to give a sound surface. Note that the new paint film over this feathered edge maintains its film thickness (Figure 3(f)). The surface of the new paint will be smoother and will hide the previous paint failure.

Ultraviolet fading
Wood that is finished with a clear water repellent, a transparent film-forming finish such as a varnish, or a semi-transparent stain will be subject to color changes due to exposure to the sun's ultraviolet (UV) rays, because these finishes, due to their transparency, allow the natural grain and color of the wood to show through. By the same token, they also allow some of the ultraviolet rays of the sun to reach the wood surface. With clear or semitransparent finishes that do not form a film, this causes the wood fibers to discolor, usually to a darker color initially and then eventually to a gray. The more transparent the coating, the more ultraviolet degradation. For film-forming clear and semi-transparent finishes, UV exposure will cause the finish to become brittle and result in severe peeling in a relatively short period of time.

Color changes due to UV exposure are difficult to reverse completely. Surfaces can be cleaned with soap and water and then treated with a wood cleaner or brightener. Several types are available; typically they contain oxalic acid or sodium percarbonate, sodium hypochlorite or calcium hypochlorite. These cleaners will return some of the color back to the wood; how much color returns is largely dependent on how weathered the surface is. Normally it is not possible to bring back the "brand new" look (this requires sanding), but a brightener can make a dramatic difference.

Figure 3. Effects of sanding to feather edges of old paint.

Blistering
Blisters can form on painted and some solid-color stained surfaces. They can be caused by temperature or moisture. Temperature blisters occur within the first few days of painting and are not a recurring problem. Moisture blisters can occur anytime excessive moisture gets into the wood or behind the coating. This moisture can come from the inside of the house or from the outside water that gets into joints and cracks. Blisters can be handled the same way as flaking, however if moisture blisters are the problem, the source of excessive moisture should be determined and eliminated in order to prevent more severe problems such as rot.

Figure 4. Paint blistering can result from excess moisture in the wood, rapid increases in surface temperature soon after painting or poor quality paint.

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

Exterior wood surfaces should be refinished only as the old finish deteriorates or for aesthetic reasons such as to change the color or type of finish. Too frequent refinishing, especially with oil-based paints and solid-color stains, leads to a thick finish buildup and subsequent cracking and peeling. Prior to refinishing, surfaces should be prepared according to the procedures described in the previous section. All mildew must be removed prior to refinishing as it can grow through the new finish.

Paint and Solid-Color (opaque) Stains
If the surface had been previously finished with a lead-based paint, be sure to follow necessary precautions for preparing the surface (see section on lead-based paint).

Latex paint can be applied to weathered painted surfaces if the old paint is clean, chalk-free and sound. The paintability of the surface can be tested with simple procedures. To test for dirt, contamination or chalking, wrap a dry sponge with a piece of contrasting colored cloth. Using moderate pressure, wipe the old surface with the cloth, using a twisting motion. Wiping a gloved hand over the surface with moderate pressure also works (Figure 5). If color is readily transferred onto the cloth, chances are the old surface is too chalky and needs additional cleaning. Perform this test in a variety of locations. If after additional cleaning some chalk still persists, the surface should be primed with a quality oil-based or acrylic latex primer prior to repainting.

Figure 5. Chalked surface as indicated by wipe test.

After the surface has been properly prepared, the best practice would be to treat areas of exposed wood with a paintable water-repellent preservative, ie; one recommended as a pretreatment for painting. Allow the water repellent to dry for at least two sunny days and then prime (Figure 6). Consult with the paint or stain manufacturer for recommended drying time. Wipe away any water-repellent preservative accidentally applied to coated areas. Apply topcoat(s). When refinishing with an oil-based coating, one topcoat is usually adequate if the old surface is still in good condition. Protected areas can often be refinished less frequently than exposed areas depending on their condition after cleaning.

Figure 6. Priming exposed wood helps ensure good adhesion and durability of the topcoat.

Surfaces that have been painted can only be refinished with a pigmented film forming coating (paint or solid-color stain), unless the original paint has been removed completely. Latex paints and solid-color stains can be applied over either oil or latex based finishes. Oil-based coatings should only be applied over oil-based finishes. Do not apply oil-based products over latex products.

Water-Repellent Preservatives and Other Natural Penetrating Finishes
Water-repellent preservatives can be renewed by a simple cleaning of the old surface with a bristle brush and application of a new coat of finish.

To determine if a water-repellent preservative has lost its effectiveness, splash a small quantity of water against the wood surface. If the water fails to penetrate and runs off the surface, the treatment is still effective. If the water soaks in, the wood may need to be refinished. Sometimes a water-repellent preservative breaks down at the surface but is still effective. In this case, the surface of the wood appears wet (dark) but the moisture does not penetrate deeply. Cleaning and/or refinishing is required when the wood surface starts to show blotchy discoloration caused by extractives or mildew.

Sound wood surfaces that have been finished with a paintable water repellent can usually be refinished with any type of finish.

Oils
Oil finishes can be renewed following the suggestions given for water-repellent preservatives.

Semi-transparent Penetrating Stains
Semi-transparent penetrating stains are relatively easy to refinish. Excessive scraping and sanding are not required -- simply clean the surface of dirt, mildew and any loose material as described above and then apply a new coat of stain. The refinish application coat of a penetrating stain often lasts longer than the first because more can be applied. Avoid multiple coats that can build up to a paint-like appearance unless recommended by the manufacturer.

Surfaces that have been finished with a semi-transparent penetrating stain can usually be refinished with any type of finish provided the surface is clean and free of water repellents.

Transparent Film-Forming Coatings
Clear varnish finishes do not perform well when used on exterior surfaces; thus, they require considerable maintenance. The refinishing practices described for opaque coatings should generally be followed for transparent, film-forming finishes such as varnish.

Removal of Finishes..........(Return to Table of Contents)

The removal of paint and other film-forming finishes is a time-consuming and often difficult process. When the process involves the removal of lead-based paints, special precautions are necessary (refer to the section Removal of Lead-Based Paints). However, finish removal is sometimes necessary for the preparation of a new surface if, for example, the old surface is covered with severely peeled or blistered paint, or if cross-grain cracking has occurred from excessive paint buildup. The removal of finish is also necessary if a penetrating stain or water-repellent finish is to be applied to a previously painted or stained (solid-color) surface. Finishes can be removed by sanding, sandblasting, mechanical removers, spraying with pressurized water, using electrically heated paint removers and blow torches, or stripping with chemicals. Consult with local equipment-rental stores and paint dealers for available equipment, or request bids from professional contractors.

Caution: Some of the finish removal procedures discussed, like blow torches and sandblasting, can be dangerous and can also cause damage to the wood surface. Some techniques require the services of a professional contractor. Always follow manufacturers' recommendations.

Sanding & Scraping
Orbital or siding sanders equipped with a tungsten carbide abrasive disk of medium grit are effective in removing old paint from smooth surfaces. This method is faster than others, and the tungsten carbide disk is less likely to clog compared to conventional sanding disks. The depth of cut for the sander can be set with the siding guide, but experienced operators often work freehand, without the guide. The operator should be careful to remove only the paint and not excess wood. After finishing with the sander, it is desirable to smooth the surface somewhat by light manual sanding or with a straight-line power sanding using 120 grit in the direction of the wood grain.

Heat
Electrically heated paint removers can be used to strip oil-based or alkyd paints. The heater simply heats and softens the paint, causing it to separate from the wood. A scraper or wide blade putty knife is then used to remove the softened paint. This method, although effective, is slower than sanding and requires at least a 1,000-W heater to be effective. Note, however, that this procedure can result in fire or dangerous fumes, hence the manufacturer's recommendations must be followed. See section on Flame removal below.

Chemicals
Commercially prepared chemical paint and varnish removers are available that remove paint from wood surfaces. They may also be effective at removing solid-color and semi-transparent stains. After the finish is removed, the surface must be neutralized before refinishing. Smooth wood surfaces should then be sanded in the direction of the grain. Strong caustic solutions, such as lye and trisodium phosphate, may leave the wood surface very porous and neutralizing and sanding are essential. Extreme care is required when using these chemicals. Refer to the chemical manufacturer's recommendations and directions.

Special Techniques for Professional Finish Removal..........(Return to Table of Contents)

The techniques discussed below are best performed by skilled professional contractors.

Wet Sandblasting and Pressurized Water Spray
Paint can also be removed by blasting with wet sand or using a high-pressure water spray. Note that neither technique should be used on paper or fiber-overlaid products or on some hardboards. Consult with the manufacturer prior to using these techniques. Wet sandblasting in particular requires the services of a skilled professional. The sand particles or water can damage and erode the wood as well as strip the paint. The softer earlywood is eroded faster than the latewood, resulting in an uneven, rough surface. These rough surfaces may not be suitable for painting.

The pressurized water spray method, shown in Figure 7, is a fast, effective method of cleaning and/or finish removal. Proper selection of a pressure washer and tip is essential. For paint removal, a washer with a pressure rating of 1,500 - 3,000 psi will be needed. A 15-degree tip is generally the most effective size for "chiseling" off paint, while a zero degree tip can gouge and damage wood. For cleaning only, lower pressures and wider tip angles are effective. Many times after pressure washing, paint edges will curl up, so it is a good idea to use a scraper on these areas prior to painting. For stubborn stains, oxidation and mildew, many pressure washers can be fitted with a chemical injector. When using a chemical injector, spray from bottom to top, then rinse from top to bottom with a 40-degree tip. Never point a power washer at anyone or get hands near the spray tip. Stay away from power lines and care should be taken when on a ladder since the initial power burst could knock you off balance.

Figure 7. Power washing can remove dirt, chalk, mildew and even paint.

Other tips to keep in mind:

1. Cover all electrical fixtures and outdoor lighting with plastic bags.
2. Close all windows and doors.
3. Cover lawn and landscape for easier clean up.
4. Protect eyes and skin and wear goggles.
5. Keep the nozzle 8 to 12 inches from the surface.
6. Use the spray nozzle like a scraper, directed at a 45-degree angle to the surface.
7. Avoid spraying directly at windows, they could break.
8. Avoid spraying into uncaulked cracks or up under the laps of horizontal lap (bevel, clapboard) siding. Do not spray into vent openings on gables and soffits.

Caution: Some old paints contain lead, and sanding, sandblasting or disturbing the surface in any way will release lead dust into the air. Inhalation of lead dust is detrimental to health. Be sure to read the section on lead-based paints. For the sake of safety, anyone sandblasting or using pressurized water spray equipment should wear approved eye goggles as well as a dust mask or respirator, as appropriate. Electrical equipment should be double insulated or equipped with a three-wire grounded outlet. Stripping of paint from the exterior of a house without proper containment of the debris can contaminate the soil if lead pigmented paints are present. In addition, debris-containing lead may be an environmental hazard that requires special disposal procedures. Consult your local disposal agency for proper handling.

Dry Sandblasting
Sandblasting with dry sand is not recommended on unfinished or finished wood surfaces because of excessive degradation of the wood surface. Alternate abrasive techniques using other materials have a limited performance history, however, some success has been reported.

Flame
Paint can also be removed with an open-flame blowtorch. A blowtorch is effective and inexpensive. However, there is a constant danger of starting a fire within the walls of the building from flames that penetrate cracks in the siding. The wood surface may also be damaged or case-hardened by flaming. This method is usually best done with extreme care by professionals. THIS METHOD SHOULD NEVER BE USED WHERE LEAD PAINTS ARE CONFIRMED OR SUSPECTED.

Removal of Lead-Based Paints..........(Return to Table of Contents)

Due to the potential health hazards associated with old lead-based paints, it is recommended to check paints for lead on houses built prior to 1978. Many governmental groups, at community levels and higher, are restricting the stripping of paint from both interior and exterior surfaces. In some instances, stripping can only be done after testing to confirm a lead-free situation or by licensed professionals. Anyone planning to strip painted surfaces should check with the local or state health department, housing or building office concerning such restrictions.

Finish Selection and Performance Expectations..........(Return to Table of Contents)

Table 3. shows the application methods and performance expectations of various types of finishes. Table adapted from the American Forest and Paper Association, "Finishing Exterior Wood Products."

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

Outdoor Decks & Porches
The horizontal surfaces of a deck or porch present a particularly severe exposure for wood and finishes and therefore require special consideration. The surfaces tend to collect water and are exposed to the direct rays of the sun so the weathering process is greatly accelerated. Any film-forming finish is subjected to excessive stress because of the continuous shrinking and swelling of the wood that results from changes in the moisture content. Furthermore, the finish is subjected to abrasive wear, particularly in high-traffic areas.

As a result of these severe conditions, a water-repellent preservative or a semi-transparent penetrating deck stain may provide the best finishing solution. Although shorter lived than paint, these finishes are easily renewed and will not show traffic patterns. They will need refinishing every one to two years.

Paints or solid-color stains are likely to peel or flake and require laborious scraping and sanding before refinishing, so unless the deck or porch is roof-covered, they are not a good choice for finishes. When they are used, treat the bare wood with a paintable water-repellent preservative prior to priming. Topcoat with a porch and deck enamel especially formulated to resist abrasion and wear.

For plywood decks, tough, skid-resistant, elastomeric coatings are available. These coatings include liquid neoprene and silicone - or rubber-based materials.

Roofs
Like decks, roofs are exposed to rain and other sources of moisture as well as intense direct sunlight. Because large quantities of end grain are exposed on shingles and shakes, the wood rapidly absorbs moisture, resulting in excessive shrinking and swelling. This moisture can be absorbed both on the exposed surface and on the backside where interior moisture migrates. As a result, film-forming finishes, such as paint, solid-color stains and varnishes should never be used on shake or shingle roofs because the result is generally severe finish failure due to peeling and flaking.

When shingles are to be finished, the first coat is best applied before the shingles are installed. Dipping is the most effective method, followed by brushing. Any additional coats may be applied by brushing or spraying after the shingles have been installed. Refinishing involves first a periodic cleaning of the roof to remove leaves, debris, moss etc. This should be followed by the application of a water-repellent preservative or semi-transparent penetrating stain formulated and recommended specifically for roofs.

Conclusion/Summary

Wood products are used in exterior applications as much for their unique aesthetic qualities as for their performance characteristics. As a result, building owners often take a keen interest in maintaining the appearance of these products over the life of the structure. This begins by selecting finishes that are compatible with the type of wood product used. Each product, be it a panel product or solid-sawn lumber, has unique properties that require consideration when selecting a finish.

Surfaces must be properly prepared before applying any finish, especially if the surface has been exposed to the weather. Continuous exposure to the elements can place considerable demands on both the wood and the coatings that are called on to protect the wood surfaces. Surfaces must be cleaned of dirt, mold and mildew, chalking, loose wood fibers and peeling paint film prior to refinishing. When applying a new finish over an existing film-forming finish, like a paint or solid-color stain, loose finish due to flaking or blistering should be removed by scrubbing, scraping or power washing.

If removal of the finish is desired, there are a number of methods that can be used. Some removal methods, like sanding, hand or mechanical scraping, chemical or heat can be done by the non-professional. Other techniques, such as sandblasting, pressurized water blasting or use of flame require the services of a professional contractor experienced in finish removal from wood surfaces. In addition, special precautions are necessary when removing older finishes that contain lead.

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