Author Topic: Why You NEVER let wood 'Weather' before Painting  (Read 10133 times)

Offline WRCLAed

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Why You NEVER let wood 'Weather' before Painting
« on: June 16, 2006, 12:18:14 PM »
Sunlight causes all wood surfaces to 'Erode'.  The longer the sun damages the surface, the faster it erodes.  

In a test begun about 23 years ago at the USDA Forest Products Laboratory (FPL), (www.FPL.FS.FED.US), various samples of Western Red Cedar siding were exposed, for different lenghts of time to sunlight.  They were all then painted with a stain blocking alkyd primer, and one coat of a 100% acrylic topcoat.  Today, 23 years later, the only sample with the paint intact and undamaged after being outdoors in Wisconsin 24/7, are the samples painted before any exposure to sunlight.  The test showed that if you applied that same paint to the same wood after it received just 4 months of sunlight, it began to peel off after just 3 years.  Today the scientists who ran these tests warn everyone that, " Any New Wood Exposed to the Elements for a period exceeding 2 weeks, has a degraded surface and is unsuitable for painting. " The longer the sunlight exposure the faster the coating peels off.  This is true for all wood.

The FPL has been around since 1910, and is part of the United States Forest Service.  They do a great deal of science with wood of all kinds, and on many different topics.  They also offer many papers with advice that is both expert and unbiased, since they do not sell anything. You are wise to consult their information whenever you deal with wood, and wood related problems.

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #1 on: June 16, 2006, 06:57:44 PM »
In 1987 the USDA Forest Service conducted a continuing study concerning the "wettability" of western redcedar panels that had been weathered for varied lengths of time. The study showed that redcedar loses most it's natural water repellency after about four weeks of exposure to natural UV ( Sunlight ). The study suggests that redcedar that has been exposed for four weeks or more to photochemical degradation should not be painted with a film forming coating. The study points out that breaks in the paint film will absorb moisture and cause the paint to fail.
Most exterior stains are not film forming coatings.
« Last Edit: June 16, 2006, 06:58:14 PM by admin »
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Ed Burke

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #2 on: June 16, 2006, 07:55:51 PM »
Actually most solid stains ARE film formers, and even when they are NOT they become film formers the moment someone applies two coats.  The advice from the FPL, (a division of the United States Forest Service), is a warning about allowing wood to 'weather' because all coatings only cling to the surface fibres that are released by the damaged lignin that has been degraded by photochemical degradation.

The key to painting ANY wood species is to apply the coating before sunlight degrades the wood surface.  Water is absorbed into all wood because it is a natural characteristic of all wood called hygroscopicity.  The difference with Western Red Cedar is its high rating for 'paint holding ability', a natural consequence of the wood and its very stable nature.  If the moisture content of the wood is at fibre saturation the paint is stressed by shrinkage when the wood drys to equilibrium moisture content.  Western Red Cedar, because it has a much lower co-efficient of expansion that most other commercial softwoods (.42) produces much less stress on the paint film in such 'worst case' situations.  However it is always necessary to prime all sides with a stain blocking alkyd primer when using solid stains or paint.  Top coats should be 100% acrylic in either case, because the acrylics are flexible enough to move with the wood and not fail over decades.  That is what is RECOMMENDED by the USDA Forest Products Lab, so it is probable your information is either outdated or inaccurate.  If you question the ability of Western Red Cedar to hold film forming finishes, how do you explain the Wood Handbook rating the species in Group 1, it's highest rated group for 'Paint Holding Ability'.  If you still doubt the fact I suggest you call the FPL and ask for R.Sam Williams,Ph.D.  Sam is the senior scientist in the wood surface chemistry work unit, among other positions he holds.  The advice I give you is verbatim, his advice.

Western Red Cedar not only holds paint well, it does it better than most other wood species.  Doing it before sunlight exposure, and with the correct prime coat is the key when using film-forming solid stains or paint.

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #3 on: June 16, 2006, 11:07:36 PM »
Quote
the correct prime coat is the key when using film-forming solid stains or paint.
Although your post seems somewhat contradictory,
I agree, that film forming paint applied to unprimed cedar will fail prematurely. Being that the surface UV exposure would be impossible to know outside the parameters of a scientific study (the real world.) We have always recommended an alkyd primer and an acrylic top coat for raw cedar, if it is to be painted. My post does NOT suggest otherwise. However, I take exception to your allusion that a true solid stain and paint have the same film forming properties. I base that not on a scientific study, but on having had the experience and pleasure of coating many thousands of square feet of raw (and UV exposed) cedar with solid stain. Inspections after years of wear show very little if any peeling. True stains are formulated to penetrate the surface of the wood not film at the surface, solid stains are usually labeled for one coat in order to prevent surface build and further mitigate the filming tendency of the acrylic binders, a tendency that film forming paint formulations promote.

Here is the study to which I referred:
http://www.fpl.fs.fed.us/documnts/pdf1993/kalni93a.pdf
 

« Last Edit: June 16, 2006, 11:47:49 PM by admin »
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the PAINTSMITH

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #4 on: June 17, 2006, 06:31:21 AM »
Not one of you discussed sanding before paint... :-[

Offline WRCLAed

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #5 on: June 17, 2006, 10:05:57 AM »
The point is you must " Sand" off the sun damaged surface before applying paint or stain that forms a paint film.  These films are capable of lasting without cracking, or peeling for decades.  That performance is wholly dependent upon the wood surface you apply them to.  If that surface has received just 2 weeks of sunlight exposure it can be unsuitable for painting.  The 'Can-Be' part of that is only related to things like direct sunlight versus a shaded area which might give you 3 weeks, or the north as opposed to the south side of the house.  But even a small exposure of the bare wood to sunlight creates the damage that causes the wood to erode, ALL WOOD.  If you apply a coating once this exists it will eventually peel off the wood as opposed to wear-off the wood. How you tell requires ONLY Real World ability, a pair of eyes working properly.  The damage the FPL warns you about occurs well before the surface turns grey, so If the Wood is Grey, it is damaged, Period!  The longer the sunlight exposure, the test indicated, the faster the coating fails.

ALL STAINS ARE CAPABLE OF FORMING A PAINT FILM !  If you apply one coat of a solid stain it is, unless it is a Semi-Solid stain, going to form a paint film.  A Semi-Solid would require two coats to form a paint film.  A semi-transparent stain would need even more successive coats, but when the pigment appears to 'gloss', bingo ! you have a paint film.  No science required.  

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #6 on: June 17, 2006, 11:21:11 AM »
The film forming attributes associated with paint are not the same as with true exterior stain, solid or otherwise. Dramatic peeling is usually not associated with stain failure, as it is with paint. However, I have seen dramatic stain peeling and "wash away" when it was applied over a primer.
If your point is that UV exposed cedar surfaces should be prepped ( sanded or power washed ) to remove surface degradation before applying a stain or pre-paint primer, then your point is well taken, and has been recommended many times in this forum.
Quote
However it is always necessary to prime all sides with a stain blocking alkyd primer
Since back priming an existing cedar structure is not a practical possibility, and based on decades of practical experience and observation, I will continue to recommend stain as the preferred coating for raw unprimed cedar on existing structures.  
« Last Edit: June 17, 2006, 09:02:00 PM by admin »
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Offline WRCLAed

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #7 on: June 17, 2006, 12:19:07 PM »
Raw, Unprimed Cedar can be successfully painted, as the test results from the FPL prove, If the cedar is primed, using a stain blocking alkyd primer, followed by at least one, preferably two top coats of 100% Acrylic solid stain, or paint PROVIDING said Primer is NOT applied to a 'weathered surface'.  That simple precaution makes the difference between 23 years of proven paint performance vs. peeling in 3 years.  Stain applied directly to the wood does not offer the same protection, and without a proper primer you will produce extractive bleeding ( in wood like cedar and redwood that has high extractive content) if the coatings are water based paints/stains which are the kind that endure the longest, and thanks to VOC restrictions appear to be the future of the coatings industry.

If you can find any reference from any science that suggests that paint films formed by the color pigments in stains has any different properties than the films created by the same exact color pigments in paints, please volunteer it.  Until that is shown I will take the advice of the scientists from the USDA Forest Products Lab that all film forming finishes behave in similar manner. That is why those scientist make the blanket statement, " Never Apply A Film-Forming Finish to any wood deck", defining film-formers as paints, solid stains, varnish, shellac, lacquers etc. That is their advice, I simply repeat it. They are the 'Experts'.  How thick that film becomes is relevant, but how it is created whether by paint or stain makes no difference.

As to the topic, " NEVER ALLOW WOOD TO WEATHER BEFORE PAINTING IT ", I am very Glad you agree with that advice.  My experience, and the primary reason I joined this forum, is that the majority of painters, whose customers call me for advice on how to 'Fix' their problem, do not understand that concept, and all the inevitable coating failures it produces.  Not surprising since most paint manufacturers spent decades advising them, on the paint and stain cans, to let the wood weather.  Most, NOT ALL, have corrected that advice.

Understanding the surface chemistry of wood is the key to having success with coatings applied to the wood.  I encourage everyone to consult the many papers available free at the FPL, www.FPL.FS.FED.US

And the FPL also says that the 'BEST' surface to apply any coating to on wood is sanded with 60 to 80 grit paper, exceeding even new roughsawn wood.  The WRCLA does not recommend ever 'Power Washing' Western Red Cedar.

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #8 on: June 19, 2006, 06:20:00 PM »
I appreciate the work being done by the FPL and your efforts on their behalf, and I have read, with interest, the recommendations resulting from the 1987 study. However, the study did not involve existing structures and the mitigating factors that they would present.
It is unfortunate that scientists do not give more consideration to the practicality of their recommendations. Back priming cedar before the application of (surface) film forming paint is an essential recommendation from the FPL. Failure to back prime the wood could result in major paint film failure (peeling and cracking) due to the infiltration of moisture through the unprimed surfaces.
Back priming an existing structure would involve siding removal and re-installation which in most cases would be cost prohibitive. While back priming is possible with new construction, architects rarely specify paint as an exterior coating for cedar and therefore do not include back priming in their specifications.

As to the differences between paint and stain. While it is true that some economy so called "solid stains" are nothing more than thinned paint, that is not true of the high end products. High quality solid stains are formulated to penetrate the wood, they are not formulated to form a hard protective film on the surface. Relative to paint, solid stains are much softer, which greatly reduces the possibility of peeling on vertical surfaces such as siding. ( Solid stain for siding is not suitable for horizontal surfaces such as decks.) Manufacturers of high quality solid stains intended for use on siding do not usually recommend a primer with it's use.
In the absence of back priming these stains can provide a good alternative to paint simply because they offer better UV protection than the more transparent stains and a real resistance to the cracking and peeling problems associated with paint.

I have always been of the opinion that painting contractors should be informed and knowledgeable about coatings and surface applications. I appreciate your input in regard to photochemical surface degradation and other findings associated with the FPL study. Knowledge and experience coupled with a degree of common sense are important ingredients in the success of any coating.  

 


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Offline WRCLAed

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Re:Why You NEVER let wood 'Weather' before Painting
« Reply #9 on: June 19, 2006, 08:38:14 PM »
I wish to take your last post item by item so we are clear in our understanding of each other.  First I am not acting on behalf of the USDA Forest Products Laboratory, though I am a committee member there, and rely almost totally on their unbiased expert advice.  I am attempting to show the causes of the problems and Dis-satisfaction with the products I represent on such issues, namely Western Red Cedar Lumber.
You could not be more wrong about the scientists who do this work.  They constructed a new house 4 years ago at the FPL just to study numerous issues, and they did not tell the contractor how to build it.  As to old homes and such issues the many branches of the FPL around the US provide a limitless supply of data about restorative issues.  You need to remember that any nationally notable structures turn to the lab, as did Howard Hughes for building his 'Spruce Goose' and other agencies like the U.S. Park Service that maintains many old, even ancient homes and buildings.  All of these experiences feed the scientist at the FPL, and they travel extensively as part of their work.  They are most definitely NOT just lab workers, far from it.  Take New Orleans, they were there before the hurricane studying what is ground zero for the worst termite threat in the nation, the Formosan Subterranean Termite.  The old homes are subject for study on many levels.  If you have paint or stain issues on older existing homes there is no one better to advise on the planet.

The advice to back prime was their advice, learned from years of dealing with such issues on wood sided structures.  They know the old siding can't be back primed, they have advice on remedial actions in such cases anyway, like putting small wedges under old clapboards to allow the water vapor an easy 'out'.

Architects are learning from seminars held in 6 different cities each year arranged by the Canadian Wood Council, 2 in Canada, and 4 in the states.  I did Pittsburgh just last month.  R.Sam Williams Ph.D. from the lab, and senior scientist responsible for this area, Wood Surface Chemistry, held two, well attended seminars filled with hundreds of architects.  I speak to architects myself on these issues, and they call my office (800 266 1910) all day long for advice and recommendations on just such topics.  The architects know how to specify paint on Western Red Cedar, and many have seen the photo's of the paint that still hasn't peeled at the FPL test fence in Wisconsin, after 23 years of exposure.

As to the paint vs, stain issue I will continue to disagree.  The modern pigments in each are trans oxide, and the acrylic formualtions are not hardened, they remain flexible, which is precisely why the FPL doesn't care which you choose to apply over the prime coat. ( the FPL says both opaque(solid ) stains and paint form paint films and therefore MUST be primed) Semi-Solid is OK to apply unprimed, BUT one coat only or you'll create the paint film.

The FPL and you and I agree about 'Film-Forming' finishes like paints and solid stains being problematic on all wood decks.

Decades of bad advice on paint cans, and yes, no input from the lumber side of the industry on coatings issues, and bogus terms like Mill Glaze to excuse what the paint companies never understood all helped mis-inform painters, homeowners, contactors and architects.  Untold millions in wasted materials, labor and coatings inflicted on American and Canadian homeowners over my entire 40 year career.  It is about time the facts were known, understood, and disseminated.  I appreciate the help.