Pre-Treatment, before priming exerior wood
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johnleeke
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PostPosted: Thu Oct 29, 2009 9:18 am    Post subject: Pre-Treatment, before priming exerior wood Reply with quote

(update: 7/7/2019 Penetrol Availability; 6/1/2015)

In painting exterior wood I usually apply a "pre-treatment" before priming and top coats. The reason I use a pre-treatment is because the paint manufacturers are changing their formulations so often (nearly every year), that we cannot possibly learn how a product performs over the long-term. I first realized this back in the late 1980s. Since then, I have separated out the adhesion-performance characteristic of my paint film system into a separate step, that I call "Pre-Treatment." This gives me more control over how well my paint film system adheres. I depend on my method and procedure for success, which is under my control; rather than depending on the manufacturers and their products, which are out of my control. Pre-Treatment is one of key ways we are able to do paint jobs that last 15 to 25 years.

I like the idea of controlling the adhesion of the whole multi-layered paint film system with that first penetrating treatment. This is part of my strategy to do best work by depending on methods, rather than depending on products. The method is pre-treat the wood surface. What it is pre-treated with is less important than the fact of pre-treatment. This way adhesion depends less on the primer product (which is out of my control since it is made by a distant manufacturer), it does depend more on the pre-treatment (which is totally under my control right in my own shop or on the worksite). I can select, mix and apply the pre-treatment to work on the wide range of wood surface conditions I work on.

Success with this method requires experience and depends on your own personal judgement. You don't know what to do until you have done it a few or several times. Knowledge is information plus experience. This article gives you the information; you have to provide for your own experience. Essentially, this means training yourself to get that experience. Begin with small test panels (one square yard), or on limited sections (say 10' wide from the bottom of the wall to the top) on the back of the building. On each panel or section do your painting procedure complete from beginning to end. Read about Christopher's experience with this:
http://historichomeworks.com/forum/viewtopic.php?p=11606#11606

There are two types of pre-treatment,

1. Paintable penetrating water-repellent preservative (WRP),

2. consolidating oil-resin (COR).

Paintable water-repellent is suitable for sound wood surfaces.
The original make-it-yourself recipe from Forest Products Lab:
www.fpl.fs.fed.us/documnts/fplrn/fplrn124.pdf
I use a modified Forest Products Lab's WRP Recipe, here is a paper on that:
https://www.fpl.fs.fed.us/documnts/finlines/finishline_mknaebe_2013_002.pdf
Here is a discussion on commercially available WRPs:
http://historichomeworks.com/forum/viewtopic.php?p=9586#9586

Consolidating oil-resin treatment is suitable for gray weathered wood surfaces or surfaces that are somewhat "soft" or more porous than perfectly sound wood. The traditional recipe for this treatment is linseed oil and turpentine. I no longer use linseed oil because it is susceptible to mold and fungus attack. I now use a mix of 1 part turpentine and 1 part oil-based alkyd-resin varnish (such as Pratt and Lambert's "38 Clear Varnish", or similar); or a mix of 1 part turpentine and 3 parts of a proprietary product (such as Flood's Penetrol, or similar (Penetrol may no longer be available)).

Application. I apply COR in the shade, and at a time when the surface will be in the shade for at least 4 or 5 hours. I apply the COR with a brush, giving the wood surface one flooding coat that may drool a bit, but not extensively. This is a case where more is NOT better. (It may be fun to watch the COR soak in, but don't keep brushing on more and more. The purpose here is to improve paint performance, not for you to have fun.) After five or ten minutes the softer areas of wood will have absorbed all of the COR, leaving a matte non-shiny surface. On the harder areas of wood the COR may still be standing on the surface leaving a wet sheen. I brush the COR from these areas over to areas where the wood is softer and will soak it up. After another five or ten minutes I wipe off all wet COR that has not soaked in. (Fire Safety: Oily rags may spontaneously combust and start a fire. Keep all oily rags in your hand or in a metal can of water.)

The whole surface should be matte, with no shiny areas. The matte appearance indicates that the surface wood cells are still open and not completely filled with COR, which would give a shiny appearance. The open wood cells are important because they provide a "key" for a mechanical bond when the oil from the primer soaks into them.

When the pre-treatment coat is 80% dry (anywhere between 30 minutes to 48 hrs depending on temperature, relative humidity, wind and sunshine), the surface is ready for a coat of oil-based wood primer. I apply the primer in the shade, and at a time when the surface will be in the shade for at least 4 or 5 hours. After priming, solvents from the COR will still be evaporating and coming out through the primer. The primer slows down the evaporation rate, so the surface may not be ready for topcoats as soon as that primer is usually ready for topcoats when no COR is used. When it doubt wait at least two weeks after priming before top coating.

It can be difficult to judge "80% dry" and there is nothing magic about the number 80, which may vary. At first the surface is wet with the oil, touch it with your finger and it feels slippery, or hold a piece of paper between your finger and the surface for a minute and the paper comes away with a dark oil spot on it. After a while the surface becomes sticky, then as it dries a bit more it is tacky. Right after it goes past tacky is the time to prime the surface. The purpose is to catch the COR while it is not yet fully cured so that the oil in the primer will chemically bond to the COR that is soaked into the wood. If the COR has thoroughly dried then the primer will not be so well attached to the wood. In this way the pre-treatment assures both a mechanical bond and a chemical bond for the primer.


Just to confuse us all, there are some combination products that may be suitable. (California's Storm Stain Penetrating Wood Stabilizer, or similar) Water-based products of both types MAY be suitable as a pre-treatment, but all my experience is with oil-based products and this method is designed for use with oil-based products.

Penetrol is an oil-based product made of mineral spirits, linseed oil and alkyd resin that penetrates deeply into the wood surface. It contains dryers (metalic salts) that significantly shorten the time it takes for the linseed oil and alkyd resin to dry. The mineral spirits evaporate leaving behind the oil and resin that cures and consolidates fibers at, and just beneath, the wood surface. Penetrol is like a light varnish or like an alkyd-resin oil-based paint without the pigment.

Storm Stain is a waterborne product that contains zinc napthanate and a very tiny amount of resins. Zinc napthanate is a preservative that limits mold, mildew and fungus. The resins help hold the zinc napthanate in the wood, but there is not enough resin to consolidate loose fibers at the surface of the wood. Storm Stain does not penetrate as deeply as oil-based pre-treatments because it is waterborne. After 24-48 hours the water has evaporated, the wood surface is dry, slightly tacky to the touch and ready for paint primer.

"Tompson's Water Seal" is a brand name that includes many products. The oil-based products that have worked so well over the past decades are now phased out and replaced with water-based products. I have not used the water-based products and I do not recommend their use with this method.


A pre-treatment may not be necessary if all the wood is perfectly sound (as with all new good-quality wood) and a very effective primer is used, but I find I can lengthen the service life of the paint coatings on new wood and old wood with this "fine tuning" of the coating system. Scientific studies at the Forest Products Laboratory have clearly demonstrated that a paintable water repellent preservative effectively adds to the protection of the wood and dramatically limits fungal decay extending the wood's life.

Research, Studies & Tech Sheets:

Restoration of Severely Weathered Wood,
R. Sam Williams and Mark Knaebe—Forest Products Laboratory, 2000
http://www.fpl.fs.fed.us/documnts/pdf2000/willi00b.pdf

Mildew and Oil-Base Paint, NPS DSC TECHNICAL BULLETIN 04- 03
http://www.nps.gov/dscw/upload/DSC-TechBulletin-04-03.pdf

Further Study:

A discussion on using epoxy and CPES as a pre-treatment:
http://historichomeworks.com/forum/viewtopic.php?p=12412#12412

Joint Coatings/Forest Products Committee Articles
http://www.historichomeworks.com/hhw/library/coatings/articles.html

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by hammer and hand great works do stand
by pen and thought best words are wrought


Last edited by johnleeke on Mon Jul 08, 2019 12:05 pm; edited 35 times in total
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sschoberg



Joined: 29 Oct 2008
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Location: Plymouth, Indiana

PostPosted: Thu Oct 29, 2009 4:43 pm    Post subject: Reply with quote

John thank you for directing my questions to the earlier procedure explanation. Your knowledge of restoration and proeservation techniques is just astounding.

What I read makes sense and am already planning on trying a pre-treatment prior tp painting on the next batch of sashes. Well, at least planning on presenting the concept to the painter/glazier (daughter #3). If she begins griping, I'm going to direct her to this forum ask her to read it for herself.

What does the turpentine and BL oil do to the Penetrol? I wonder if the formula for Penetrol is changing to conform to the new regulations, just like paint? I guess I can do some of this research myself, huh?

Do you have or does anyone else have any side by side results? Pre-treatment versus no pre-treatment?

So, (I'm thinking outloud and scratching my head) the pre-treatment actually penetrates into the wood further than the oil primer would on its own. Then the oil primer has something to grab hold of, with better results than by itself? Is this it?

Since the pre-treatment gives something better for the primer to hold to, is anybody using a fast drying oil primer instead of regular oil primer?

Is applying a pre-treatment prior to priming being done by a majority of quality minded restorers?

Steve S
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rncx



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Location: Little Rock, AR

PostPosted: Fri Oct 30, 2009 12:39 am    Post subject: Reply with quote

basic properties of paints...

1) the oil in a paint is just a vessel for the pigments. once the paint cures the oil is gone. at this point oil paints start to get brittle. that's what causes the cracking and flaking you see on old oil paint jobs. acrylic paints are more flexible and not as susceptible to this. so acrylic paints are arguably more suitable for exterior surfaces when comparing modern paints. problem is acrylic paints don't stick well/at all to previous oil paint layers. so if you're painting over oil, there's no way to know until it fails, and if it does fail it will fail miserably. there's also the question of abrasion resistance on a window. enamel paints are generally more durable in terms of abrasion, which is why people still use oil enamels for doors and windows.

2) wood is thirsty. old wood that has had a century or more to lose all of its sap is even more thirsty. the faster the oil is removed from the paint, the faster it will fail. if the wood is absorbing a significant portion of the oil from your paint before the paint cures, that accelerates the failure time.

3) wood is not consistent. certain parts of a tree, and therefore certain parts of the grain in a board, absorb much more than others. you can see this any time you try to apply a stain to unsealed pine. it gets splotchy, because certain parts absorb stain better than others. the same thing will happen with paint over unsealed wood. certain parts of the wood will absorb more, certain parts will absorb less. the result will be inconsistent drying time over the surface of the primer, inconsistent adhesion over the surface of the primer, etc.

that's why when you talk to anyone who finishes wood for a living, they'll tell you that regardless of the finish the first coat is always a sealer. you have an inconsistent and absorbent surface. the only way to solve that problem is to apply a sealer first to make the surface consistent. the only other way to improve the consistency of a finish is to spray the finish with an air powered sprayer. spraying with air also causes a finish to dry more quickly, and thus, evenly, preventing the wood from over absorbing. that doesn't necessarily relate to a more durable finish, mind you, just a more consistent one.

that's the catch to getting mirror-like gloss finishes on furniture and such in lieu of methods like french polishing, fwiw. you use a very fast drying finish, and spray it thin with air so that it dries almost on contact. that's why furniture is finished with lacquers (and previously, shellac). they dry super fast. you can lay down a flawlessly even coat with an air sprayer very quickly, and after you buff it smooth, you're left with a very even finish.

on the topic of point 1), that's why i'm really optimistic about the allback paints. the obvious problem you see from the properties of modern paints are...

oil is durable, but is basically a plastic coating. it's a metallic pigment (coated typically with silicone because the metals we've replaced lead with are not durable) in an oil. so when the oil is gone, the entire coating falls off. acrylics don't suffer as much from this, their pigments are different. they are typically quartz and talc. so they are not affected by sun and water as much as the metallic pigments in oil paint, but, they are still different from old style lead/linseed oil paints in one basic way. once the vessel is gone, in the case of modern acrylics the vessel being water, water won't have any affect on the paint. the binder is not the water, the binder is separate from the solvent. as the binder fails the pigments chalk off, like old lead paint oxidizes over time, but the million dollar difference is the lead paint was bound with a simple oil, which could be re-introduced into the coating, thus preserving the coating. you can't reintroduce more binder to an acrylic coating, you can only paint over it with a new coat of paint, which is simply adding more layers to an original layer that was bound to a primer, not the surface being painted. so no matter which style of modern paint you choose, at some point in the future you are faced with complete removal.

so those are your choices with modern paints...

1) an oil that will fail by separation, never a good thing, but is resistant to abrasion.
2) an acrylic that will fail by pigment loss, which is a good thing, but that can't be maintained without new layers and washing of loose pigment, because you can't re-introduce the binder

traditional linseed oil paints are/were basically different in that way. the binder is the linseed oil. and there is no primer. the linseed oil penetrates into the wood, deeply. therefore there is a significantly longer period of time before a complete failure of the coating, because, as we know, oil paints fail when the oil is gone, and it takes alot longer for a linseed oil sealer + a linseed oil paint to loose all of its oil. and the key difference with old linseed oil/lead paints is you can reintroduce more of the binder into the coating and the surface being coated, because the binder is a simple readily available oil.
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sschoberg



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PostPosted: Fri Oct 30, 2009 6:14 am    Post subject: Reply with quote

rncx,

Wow, I understand. Thank you!

Steve S
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johnleeke
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PostPosted: Fri Oct 30, 2009 8:18 am    Post subject: Reply with quote

Here is another 'take' on how paint works, over in the Historic HomeWorks Library:

http://www.historichomeworks.com/hhw/qa/qa07.htm

also in the library, the Coatings Committe articles:
http://www.historichomeworks.com/hhw/library/coatings/articles.html

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sschoberg



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PostPosted: Sat Oct 31, 2009 8:21 am    Post subject: Reply with quote

Thanks John.

Again your forum is proving its value. A wealth of knowledge, from a smorgasboard of experience, released with a healthy dose of passion.

Truly worth much more than the price of admission.


Steve S
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johnleeke
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PostPosted: Sun Nov 01, 2009 10:39 pm    Post subject: Reply with quote

Quote:
Truly worth much more than the price of admission.


Steve, I have been thinking of providing an opportunity for folks to make a dollar contribution to the website. How would you respond to that?

I'm not suggesting that you should. I think that if you have found value here that you have also provided a significant value with your participation.

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waxahatchie



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PostPosted: Mon Nov 02, 2009 1:15 am    Post subject: donations Reply with quote

john, you should; i bought your publications, but not everyone needs them. the info provided here is legion - and the upkeep is not free. if you put up some sort of donation page, it would help with the housekeeping fees.

it would be a tragedy if the site went dark due to bandwidth charges getting too high, or some such!
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sschoberg



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PostPosted: Mon Nov 02, 2009 8:08 am    Post subject: Reply with quote

Steve is contemplating your first question-----------while scratching his head.

And thanks!

Steve S
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johnleeke
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PostPosted: Sat Nov 07, 2009 4:00 pm    Post subject: Reply with quote

Quote:
it would be a tragedy if the site went dark


Don't worry, no chance of that.

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Scott S



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PostPosted: Fri Dec 28, 2012 1:14 am    Post subject: Pre-treatment on windows sashes Reply with quote

I have never used this method before but it really sounds intriguing to me. I'm really keen to try this on some of our projects.

My question is: Have you tried this on wood windows sashes? Have you had any improvements or problems with the glazing putty in regard to using a COR pre-treatment?

I prime all my sashes after sanding with an oil-based primer prior to glazing (even though I notice that the rebates were rarely primed originally). I assume that instead of priming they were prepped with linseed oil instead? Curious to hear anyone's thoughts.
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johnleeke
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PostPosted: Fri Dec 28, 2012 9:02 am    Post subject: Reply with quote

Scott, welcome to the Forum!

Yes, I've routinely used pretreatments on wooden window sashes for three decades. It improves durability, and has not caused putty problems in my experience. Here's the low-down on how to use it when painting and glazing window sashes:
http://historichomeworks.com/forum/viewtopic.php?t=955

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Scott S



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PostPosted: Sat Dec 29, 2012 1:07 pm    Post subject: Priming sides of sashes Reply with quote

John, thanks for the wealth of info in your reply. I do have a question regarding priming the sides of the window sashes. Why do you leave those raw wood. I would think that in a very humid climate like we have in Florida it would be better to prime the sides to prevent the sash from swelling or shrinking too much with weather changes.

I've been having my team do that and I'm concerned now that I might have problems down the line. Thoughts?

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johnleeke
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PostPosted: Sat Dec 29, 2012 3:51 pm    Post subject: Reply with quote

It has always been general practice to leave the side edges of double-hung sashes bare wood so that the sash can slide easily in the tracks (also usually left bare wood), and so that the wood can dry out (promoted by air infiltration and ex-filtration past the edge), especially at the joints, keeping them dry and decay free.

This has proven to work well in temperate climates for over four centuries. It could be that in localized tropical climates on the southern fringes of the South the performance of this detail may vary.

One basic of keeping wood in good condition in all climates, is to assure that it can dry out when it does get wet. In some situations primer would tend to keep moisture in the wood, and not let it freely dissipate from the surface of the wood.

Another basic is that local materials often perform better than materials from away. When in the tropics, use wood from that climate, which is more likely to be decay resistant, such as Cyprus or Mahogany.

The best practice is always guided by observations of what works in your local, and has actually proven to last over the long term, for decades and centuries. How many sashes do you see that are over a century old, that have bare wood edges, and how many with primer or other coatings on the edges?

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johnleeke
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PostPosted: Thu May 02, 2013 7:01 am    Post subject: Reply with quote

I started developing these pre-treatment methods back in the 1980s, based on studies at the Forest Products Laboratory and my own experience.

If the paint is lifting off down to bare wood the main cause is excessive moisture in the wood. The source of the moisture may be from inside the house, or from rain water seeping in through joints in the woodwork, or through cracks in the paint film.

Extreme Prep:
If the existing paint film is thicker than .015" (a dime is .023" thick), then it is thick enough that the water vapor cannot easily escape from the wood directly through the paint film, so it simply pushes the paint film off the wood on its way out. When we want to get the maximum durability of the new paint coating we remove all the existing paint down to bare wood. We also clean the wood with a Wet Abrasive Scrub:

http://historichomeworks.com/forum/viewtopic.php?t=1751

Durability:
With the combination of 1. Removal of heavy paint buildup, 2. Wet Abrasive Scrub, and 3. Pre-Treatment, plus experienced and skilled workers we have been able to get 20 to 30 years of life out of a paint job. With an extreme salt water micro-climate that drops down to 15 to 20 years.

Sealing the Wood:
>>It seems to me that sealing the wood would reduce the opportunity for the primer to soak into he wood for more "bite."<<

An effective pre-treatment should not seal the wood. It should leave most of the wood cells at the surface open so the primer can still make a mechanical bond by soaking into the cells. About 40% of the bonding is mechanical and 60% of the bonding is chemical. The pre-treatment improved chemical bonding by making the highly variable surface conditions of old wood more consistent and suitable for better performance of the primer.

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