# Paint failure on cedar shake shingles



## RCP (Apr 18, 2007)

Interesting article on a case of paint failure on D & D this morning. Here is the link, but you have to register to read full article.
Here is a copy,


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## DeanV (Apr 18, 2007)

Interesting article. They reference the Forest Product Labs recommending 100% acrylic products for use on all exterior wood. I have read the Forest Product Lab wood finishing sheet once and they even recommend the acrylics for the first coat on raw exterior wood.

This flies against conventional wisdom for most painters, especially on cedar. I tend to prefer and oil primer followed by acrylic stain personally. 

This article is another reason why pre-finishing is a great idea. Get that wood protected before it can degrade. I cannot believe on homes like those that a GC is willing to forgo back priming and also let a home sit unfinished for that long (3-4 weeks). Water staining on cedar alone is reason not to leave it unfinished and exposed.


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## Bender (Aug 10, 2008)

> The field testing and laboratory results indicated that the underlying cause for the paint failure was a deteriorated layer of
> wood beneath the paint.


That seems a bit fishy to me.


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## oldskool (Nov 17, 2009)

Bender said:


> That seems a bit fishy to me.


It shouldn't, 
the lignin in the wood will break down as water trys to leave the wood surface. the paint film will block the exit of the moisture and as it sits there in that small area, the "glue" that holds the wood together breaks down and adhesion to any coating at that point is compromised.

the newer modified acrylics have excellent adhesion and flexibility. both primers and stains. this has become our choice of exterior wood finish, though they tend to be more money. Oil was best but they are so comprimised because of VOC restrictions that they don't have the penetration into the wood to help create a great bond.


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## Bender (Aug 10, 2008)

oldskool said:


> It shouldn't,
> the lignin in the would will break down as water trys to leave the wood surface. the paint film will block the exit of the moisture and as it sits there in that small area, the "glue" that holds the wood together breaks down and adhesion to any coating at that point is compromised.


Not much moisture in an alkyd


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## Bender (Aug 10, 2008)

I would lean toward high moisture content prior to paint.


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## oldskool (Nov 17, 2009)

not back priming would be a cause of alot of moisture intrusion over the changing seasons also. 

Sealing all six sides is ideal, if the wood is dry to begin with.

we have also seen 2 x 6 boards painted with elasto, rot from the inside, and snap in two in about 5 years.


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## DeanV (Apr 18, 2007)

In the literature for finishing, they make a pretty big deal about coatings not adhering to cedar that is exposed to sunlight for few weeks. It seems to be pretty well documented.


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## TJ Paint (Jun 18, 2009)

hmm lets see...

sounds like the weathered wood wasn't properly prepped before finishing. thats a shocker.


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## oldskool (Nov 17, 2009)

So in the article it says essentially " thick coats of oil based paint with a low resin content failed worst on the south sides" seems pretty straight forward. hard film on a flexible surface = failure every time. with that substrate and that product type repaint every 5 years would be expected.

What about painting over a cedar that has a mill glaze? unweathered wood can cause the same issues in this article if you use the same oil products on them. the wood hasn't changed, paint formulas have.


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## DeanV (Apr 18, 2007)

It would be interesting to know if it was all sprayed coats or brushed in coats as well. Especially the first coat. With some of the really thick millage amounts, I am guessing spraying was involved. Brushing could also do that, but I think it would be in very limited areas. It probably would not show up in sampling unless they purposely took a sample from a drip area from where paint ran down between shakes or something like that.


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## plainpainter (Nov 6, 2007)

reason why paints fail so much is due to the fact that most modern lumber is a sponge ready and willing to soak in whatever moisture it can. The more moisture a piece of wood can soak up - the more dimensionally unstable it becomes. The more dimensionally unstable - the more your coating better be able to flex. Oil coatings of the past were so successful has more to do with the dimensional stability of old growth forest products than the actual paint. Say for example you use either Mahogany or Spanish Cedar wood for your trim - once you do an acetone wipe down of the wood - properly primed and painted, it will last forever.


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## Bender (Aug 10, 2008)

> So in the article it says essentially " thick coats of oil based paint with a low resin content failed worst on the south sides" seems pretty straight forward.


And that makes sense, but you would expect it to be a more common problem. Most homes are done in alkyd (in that region) with newer low VOC product, and new growth wood.


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## Bender (Aug 10, 2008)

On a side note: We had to call KTA Tator out to a project one time. Pretty neat people


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## straight_lines (Oct 17, 2007)

Ever since acrylic stains have been dominant I have always put a coat of stain on first. If any knots bleed then come back and hit them with oil before the finish coat. Oil is just to brittle once it cures out, there is no flex at all.


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