How to prepare a wood surface for a successful, durable paint job.

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

Photo above: here we see a complete loss of adhesion of large areas of new paint under investigation by the editor [DF] - even the best-quality paint will fail to adhere to a poorly-prepared surface.

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 PHOTOS, 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.

[Click to enlarge any image]

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.

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.

8-Step Preparation of Old Painted Surfaces to Receive New Paint

Photo above: severely alligatored paint surface. You can see that recent paint bonded to the "cleaned" area iof this wood clapboard, but more paint is peeling off.

Failure to remove old paint like this means your new paint job will fail quickly.

1. Avoid common painting mistakes:
   
   Do not fail to read the catalog of ways to foul up a paint job listed
   
   at PAINTING MISTAKES. Examples include painting in hot sun, painting a wet surface, and poor surface preparation.
   
2. 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.
3. 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.
4. Remove alligatored paint
   
   (such as in our photo above).
5. Remove cracked paint.
   
   Similar to alligatoring but less extreme-looking, cracked paint on a surface has lost its elasticity and will quickly crack through the new painted surface.

Photo above: during a consult concerning paint failure on a building we pionted out the effects of failure to adequately feather edges of areas where old paint remains.

6. Feather edges of old paint:
   
   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.
7. Use a primer:
   
   on new wood use a suitable primer including, where the wood contains heartwood or knots or other water-soluble extractives, using a primer labeled by its manufacturer as "extractive bleeding blocker".
   
   Give strong consideration to using a primer to improve adhesion between the old painted surface (and any bare wood) and the final top coat.
8. Apply the finish coats of paint:
   
   by actually reading and following the manufacturers instructions.
   
   For wonderfully-detailed painting advice see
   
   Chapter 16, Finishing Wood, in WOOD HANDBOOK [PDF] U.S. Forest Products Laboratory, (2010) retrieved 2021/06/19 original source: https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf
   
   At PAINT FAILURE, DIAGNOSIS, CURE, PREVENTION you'll find a description of exterior conditions that can ruin a new paint job like painting over a damp surface, or in hot sun.

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , content expanded from US FPL Wood Handbook & by InspectApedia.com editor.

This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , by Steven Bliss, courtesy of Wiley & Sons.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Should I prime my wood before assembling it into a door?

Should I prime T and G pine wood before assembly into a door ?
Then paint after assembly On 2020-08-06 by Les Tomlinson

by (mod) - prime all sides of wood

Great question,

in my opinion priming all of the sides of the wood will reduce moisture uptake and make the door more durable because you're protecting the hidden sides from moisture uptake.

Daniel Friedman

What kind of stain should I use on a weathered board and batten spruce sided house?

I have a spruce board and batton house that I built in 2014.

It has never been treated and parts are turning grey.

I think best bet is to use solid stain to cover up the discoloration, but what should I use before I stain ..Wrp ?

If so what is a good brand. I live in northern maine close toCanadian boarder.
Tks ! On 2018-05-19 by CATHY

Reply by (mod) - for uneven colors you might want a pigmented, penetrating wood stain

As you may know weathering to a gray color is considered normal.

If you don't like the color variation in these weathered boards, then I agree that you may want to apply a good quality preservative stain that contains a pigment.

With that product you shouldn't need a primer.

Keep in mind that even applying the staain the colors are not going to be exaxtly uniform on your house.

But stains with higher levels of pigment will hide most variations on the original surface.

Watch out: I would also be careful about power washing as you can raise the wood grain, drive water into the wall and damage the siding or create a longer dry-out period before you can apply the stain.

 

Question: plans for using an alkyd paint on an outdoor deck

I am going to re-paint a outdoor deck.

In the N.W. Half the deck is exposed to sun, rain, snow and serious dew.

After reading the information here, I am going with a alkyd paint. Unless someone suggest otherwise.

The current coating has good adhesion and no cracking. But it is weather damaged from rain and sun. (July 11, 2015) old school said:

Reply:

Old

I'm fine with alkyd paints,

Just watch for creating slippery surfaces.

How do I dry out a board at wall bottom to prep for painting - need a quick fix

I have garage trim board that has wicked up moisture about 4 to 5 inches up off the driveway.

Have sanded, primed and painted in the past (the bad spot) and it continues to (after a while) bubble the paint and then the paint peals off.

I am not ready to replace the board as it involves removing garage door trim, etc. Need a quick fix on how or what to use to dry out that portion of the wood before I prime and paint again. Thanks. 3 Novemb er 2015 Ssusan said:

Reply: superficial drying of wood won't safe your paint job

When weather permits, perhaps a box fan blowing along the board.

Watch out: But frankly if the board is at ground or in ground contact it's not going to be durable and there's a risk of hidden insect attack and rot.

When I've tried using a fan, a heater, a hair dryer, or other methods to dry wet wood I have found that usually just the surface of the wood is actually dried; moisture remains inside the wood and takes longer to dry out,

So what I do is to simply leave that board unpainted until it has had more time to dry, and I check it with a moisture meter, not just by surface touch,

Thank you to our readers for their generous comments

On 2020-07-08 by ASH Green

ASH Green said:

One of the best articles that I’ve read in a very long time! I Took notes and surely gonna implement and test bunch of stuff you talked about

You’re a beast! Cheers,

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Continue reading at PAINT / STAIN SELECTION & PROCEDURES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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• PAINT FAILURE DICTIONARY - illustrated list of all types of paint failures
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