How to Prepare a Wood Surface for Exterior Paints or Stains

How to prep walls & trim for painting
- Exterior Finishes on Wood: moisture, surface prep, paint choices, solid color stains, paint & stain application details, discoloration, semitransparent penetrating stains, clear & light-tint finishes, bleaching oils, unfinished siding & trim
Questions & answers aboutsurface preparation to assure a durable paint job
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Preparing a wood surface for successful painting: this article provides details about wood surface preparationbefore paint or stain application in order to assure a successful job. A successful paint or stain job means that the result looks good and is durable.

How To Prepare a Wood Surface Before Painting or Staining

This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains. This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

For a detailed guide to selecting and using exterior paints and stains, readers should also see PAINT & STAIN GUIDE, EXTERIOR. Also see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION and PAINTING MISTAKES for details of paint failure cause, diagnosis, cure and prevention. Odors from paints and low-VOC or zero-VOC paints are also discussed at ODOR DIAGNOSIS CHECKLIST, PROCEDURE. Also see Volatile Organic Compounds VOCs.

Failure to properly prepare a surface to receive new paint is the most frequent cause of paint job failures. We discuss this problem at PAINTING MISTAKES. Also take a look at our field investigation of an expensive paint job that failed for just this reason, at Paint Failure Case Photographs-SITE. Rather than clean and strip thick, old alligatored (and lead-containing) paint from an older building surface, the low-budget painter simply skim-coated the entire building with a crack filler and then painted over that. The paint job failed so fast that paint was falling off of the building walls even before all of the walls had been painted.

How long a finish will last depends on many variables, including the quality of the finish, type and texture of wood, application conditions, and exposure. South- and west-facing walls get the most sun and are, therefore, often the first to need recoating. Whether painting, staining, or finishing in any manner, the FPL makes the following recommendations:

Make Sure of Proper Wood Moisture Content Before Painting

Never paint wood with a moisture content over 20%. Ideally, the wood should be painted at its average moisture content for that climate - about 12% for most of the United States, 9% for dry southwestern states (see Table 1-2). At Moisture Impact on Paint we provide additional details.

Table of wood moisture content on buildings (C) J Wiley, Steven Bliss

New Wood Surface Preparation For Painting

A rough-sawn wood surface will hold paint and stain much longer than a smooth, planed surface, which is why many contractors prefer to install siding rough side out. Also most lumber and siding today is flat-grained, which holds paint less well than vertical (or edge) grained.

The combination of flat grain and planing can create a burnished surface called “mill glaze,” which can cause problems with paint adhesion. To avoid problems, it is best to lightly sand with 50 to 80 grit sandpaper before painting smooth siding. The optimal approach is to first wet the lumber to raise its grain and then let it dry for two days before sanding.

Allowing Wood Siding to Weather Before Finishing

Some painters recommend letting smooth siding weather for a few weeks to open up the grain. However, research at FPL has shown that after two weeks of exposure, the wood surface begins to degrade and to loosen the wood fibers on the surface, which weakens the paint adhesion.

The FPL therefore strongly recommends painting within two weeks of installation, whether the rough or smooth side is facing out. If you need to paint wood that is badly weathered, the wood should be sanded, power rinsed, and allowed to dry before priming. Once the primer is dry, the top coat should be applied as soon as possible.

Wood Species and Wood Grain Affect Paint Success

In general, less dense woods hold paint better than more dense woods (see Table 1-12). Also, within a single species, vertical-grain (also called edge-grain) wood holds paint much better than the more common flat-sawn lumber, primarily because flat sawn wood shrinks and swells more from changes in relative humidity.

Table of wood finishing characteristics of different wood species (C) J Wiley, Steven Bliss

Also vertical-grain wood has narrower bands of latewood, the denser and harder portion of each annual ring in a tree. When paint, particularly oil-based, becomes brittle with age, it tends to peel from the latewood.

Dense woods with wide, flat grain will present the greatest problems in holding paint. This is true for most hardwoods as well as dense softwoods with wide, flat grain, such as southern yellow pine and Douglas fir, especially if planed smooth.

Clean the Wood Surface Before Application of Paints / Stains

The best paint in the world can fail within the first year if applied over a wet, dirty, or degraded substrate. So the first priority is to make sure that the material being painted is sufficiently dry and clean.

Temperature and Time of Day Impact on Paint Job Success

Oil-based paints should be applied when it is over 40°F; for latex coatings the temperature should be at least 50°F during application and for 24 hours after. Also it is best not to apply paint too early or too late in the day. If the dew has not evaporated in the morning, both oil and latex may have adhesion problems. If applied within two hours of sunset and a heavy dew forms before the paint dries, latex paints may streak and oil-based paints may not cure properly.

Use of Sealers on New Wood Before Painting or Staining - Water Repellent Preservatives (WRPs)

For the best protection of the underlying wood and the longest lasting finishes, bare wood should be sealed with a water-repellent preservative (WRP) before priming and painting or staining. WRPs contain a small amount of wax or other water repellent and a mildewcide, fungicide, or both, usually in a solvent base. The preservatives help prevent mildew and decay in above-ground applications but are not meant for ground contact. Some WRPs contain UV blockers as well, which slow down the degradation of the outer wood fibers.

While sometimes formulated as a finish treatment for siding, some WRPs can be used as a pretreatment for painting and are recommended for that use by the USDA Forest Products Laboratory (FPL) and Western Wood Products Association (WWPA). Research shows that WRPs resist water entry better than acrylic primers. On bevel siding, they also reduce warping, splitting, and mildew growth. They can also improve paint performance on hard-to-paint woods, such as southern yellow pine and Douglas fir.

In new construction, the FPL recommends that siding and trim be coated on all sides with a paintable WRP such as DAPWoodlife® or Cuprinol’s Clear Wood Preservative, preferably by dipping or with a brush, roller, or pad. If the siding or trim is already installed, they suggest treating all places vulnerable to water entry, including door bottoms, window sills, lap and butt joints, edges and ends of trim, and any end grain on panel products such as plywood sidings.

If used as a pretreatment for paint, apply to bare, dry wood when it is above 50°F, and use only a single coat or excess wax buildup on the surface could affect the paint adhesion. Allow two days of warm weather to dry, or up to a week if the material was dipped. If painted before the solvent has evaporated and the wax absorbed, the paint can be discolored and not bond well.

Importance of Priming New Wood Before Painting or Staining

All paints and most solid stains require priming on new wood. Primers are formulated with a higher ratio of binder to pigment than paints. [This means that there is more binder and less pigment in a primer than in a finish-coat of paint - Ed.] The primer forms a durable film that bonds well to the surface and blocks water. However, without much pigment, it offers limited UV protection.

For woods with water-soluble extractives, such as cedar and redwood, use an oil-based primer or a stain blocking acrylic primer formulated to seal in the extractives. Also use a stain-blocking primer on any knots. Otherwise the extractives can bleed through the finish and stain the siding. For wood species relatively free of extractives, use a 100% acrylic latex primer. If sprayed or rolled on, back brushing is recommended for a good bond.

Many manufacturers now sell siding and trim preprimed. In addition to the convenience for the contractor, the factory-applied coating is applied uniformly without the risk of bad weather or other job-site variables. The only concern is the thickness of the primer. While most major manufacturers of preprimed siding do a good job, some third-party pre finishers may ship material with too thin a coating. In general, the primer should be 1.5 to 2 mils thick—thick enough that it hides the wood grain.

Use a Paint or Stain Primer Compatible with the Paint or Stain Top Coat

Watch out: as we discuss at PAINT / STAIN SELECTION & PROCEDURES, be sure that the primer you select for your paint job is compatible with the type of coating you intend to use for the top-coat. Read the manufacturer's label-instructions and check with your paint supplier. Painting some top coats atop the wrong primer is a guarantee of a failed paint job that shows up as wrinkling, cracking, or loss of adhesion of the building coating.

For example, if a solvent in the top coat is incompatible with the primer coat, it may cause the primer to dissolve, lose adhesion, or wrinkle. And if the chemistry of a primer is not compatible with a top coat, the top coat may simply fail to properly adhere to the primer.

We like alkyd paints because of their compatability: alkyd-based paints can generally be applied over either older oil-based painted surfaces or over latex based paints, reducing the need to strip otherwise sound paints from an older building surface. If you don't know the chemistry of the paint on a building exterior, using an alkyd primer will help assure a good bond to the old surface, even if your final top coat is going to be latex.

Finally, don't use an indoor-rated paint on a building exterior. Interior paints not intended for outdoor use may lack adequate UV protection, weather resistance, or a binder and adhesion necessary for success on an outdoor surface exposed to the weather. - Ed.

Role of Back Priming Wood Building Siding or Trim

Most paint failures are related to moisture moving through the wood either from wind-driven rain that reaches the back of the siding or moisture escaping from the house. In some cases exposed end grain picks up moisture and causes localized peeling. Use of a water repellent preservative or primer on the back of the siding and on all edges and cut ends, in addition to the visible face, will minimize these problems. Sealing the wood properly also helps prevent moisture from being driven through the siding by solar radiation.

When to Apply Paint Top Coats on Building Exteriors

For paints and solid stains, apply the top coat as soon as the primer is dry but not more than two weeks later. For best performance, apply two top coats. Latex paints can typically be recoated within a few hours. Oil must cure for one or two days between coats. Apply paint at the coverage recommended on the can. Too thin a coat will wear quickly and too thick a coat may crack.

While brushing provides the best adhesion, a properly done spray job can yield good results. When spraying or rolling, the best results are achieved by back-brushing the paint to help work it evenly into the wood, particularly on rough-sawn surfaces.

Tips for Painting or Staining Pressure-Treated Wood: CCA, AZCA, ACQ Treatments

Wood that is pressure-treated with waterborne preservatives, such as chromated copper arsenate (CCA), ammoniacal copper zinc arsenate (AZCA), and ammoniacal copper quaternary (ACQ), present special problems for painted finishes. First, pressure-treated lumber is often shipped to lumberyards with very high moisture contents. If painted while wet, the moisture may get trapped by the paint film and cause peeling. Also the species most commonly pressure treated— flat-sawn southern yellow pine in the eastern United States and Douglas fir and Ponderosa pine in the West—do not hold paint well to begin with.

Whether or not you intend to paint the wood, pressure treated exterior trim should be sealed with a water repellent preservative as soon as the surface is sufficiently dry. This will protect cut ends and help keep the wood from checking, cupping, and warping as the wood dries out. If this is the only treatment, it will need recoating every one to two years. Factory-sealed treated lumber is now available that only requires treatment of cut ends when installed.

The most common treatment for pressure-treated wood is an oil-based, semitransparent stain. Since this type of finish is relatively permeable to moisture, for best results apply it over a sealer or over factory-sealed lumber. While the sealer can be applied to wood that is still wet inside, it is best to air dry the wood before staining. This will take from a few days to a few weeks, depending on conditions, with two weeks on average. Two coats of an oil based, semitransparent stain over a sealer should last several years. The second coat should be applied before the first coat dries completely, or the second coat cannot penetrate the wood.

If a painted finish is desired, you will need to seal the wood first and allow it to dry for two to three weeks before applying a compatible primer and two coats of 100% acrylic top coat. The longer the wood dries, however, the greater the risk that UV radiation will damage the wood surface, interfering with the paint’s adhesion. To avoid these problems and the long delays, consider using kiln dried treated lumber that can be finished immediately.

Preparation of Old Painted Surfaces to Receive New Paint

Do not fail to read the catalog of ways to foul up a paint job listed at PAINTING MISTAKES.
Then at PAINT FALURE, DIAGNOSIS, CURE, PREVENTION you'll find a description of exterior conditions that can ruin a new paint job.
Clean the building surface. If you use power-washing, don't spray water up under clapboards or siding where you'll soak the building wall cavity, and be sure that all surface areas are dry before going to the next step.
Remove loose paint. Watch out: if the age of the building means it has paint applied before 1978 there is almost certainly lead paint present, and you'll need to take proper precautions.
Remove alligatored paint (such as in our photo at left).
Remove cracked paint. It has lost its elasticity and will quickly crack the new painted surface.
Where loose paint has been removed, sand or use a sharp scraper to feather the edges of remaining old paint that is sound and is being left in place.
Give strong consideration to using a primer to improve adhesion between the old painted surface (and any bare wood) and the final top coat.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Suppliers of Exterior Paint & Stain Products

Water-Repellent Preservatives (WRPs)

Cuprinol www.cuprinol.com Cuprinol Clear Wood Preservative

Dap www.dap.com DAP Woodlife

Wolman www.wolman.com Premium Water-Repellent Sealer

Clear Wood Finishes

Amteco www.amteco.com Total Wood Protectant (TWP)

The Flood Company www.floodco.com Clear Wood Finish (CWF)

Performance Coatings Inc. www.penofin.com Penofin wood finishes

Sikkens/Akzo Nobel www.nam.sikkens.com Sikkens Cetol finishes

For More Information on Building Practices for Exterior Wall Products

California Redwood Association www.calredwood.org

Cedar Shake and Shingle Bureau www.cedarbureau.org

USDA Forest Products Laboratory (FPL) www.fpl.fs.fed.us

Vinyl Siding Institute www.vinylsiding.org

Western Wood Products Association (WWPA) www.wwpa.org