Technology Tips from the Paint Lab: Making Paint Stick to Weathered Wood

From D+D Online, September 2011

by V.C. Bud Jenkins

Painting over weathered wood essentially involves the same processes and procedures as painting over rusty steel, or painting over a wet surface, or painting over a dirty, sandy surface....
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Tagged categories: Coating chemistry; Coatings Technology; Restoration; Wood; Wood coatings

Comment from Chris Murphy, (9/16/2011, 8:24 AM)

Excellent article! Great step-by-step explanation of the links of the chain that need to be in place to ensure a successful coating. Thanks!

Comment from Catherine Brooks, (9/16/2011, 9:21 AM)

This one is the answer to "I've always wanted to know....why old, painted wood is so hard to make look nice and to make stay that way." It will take me a few reads to understand it all, but thanks for this sophisticated level of detail. Our customers are getting pretty savvy and the more we can explain ourselves, the more credibility we gain.

Comment from David Reynolds, (9/16/2011, 10:25 AM)

Kudos for the explanation of what is in the debris that builds up on wood outdoors, and about cellulose and lignin as a composite material. In a recent article in American Painting Contractor (88:6, July 2011, pp 48-49) I address a less complicated worn wood problem outdoors, where only a clear oil finish is involved. In that case, dilute oxalic acid or percarbonate (mild oxygen bleach) cleaners, worked with a fine, stiff brush and rinsed gently, remove the accumulated breakdown and airborne materials from the wood surface. The essential thing is to expose clean, sound wood to penetrating and sealing coatings. Have you and your group addressed the surface chemistry and physical effects associated with infrared tools such as Speedheater, which save managing runoff water when RRP applies, and can help with built up paint and debris on wood detail? Thanks

Comment from Ken Johnson, (9/16/2011, 10:27 AM)

As a heritage consultant I am often faced with the dilemma of painting weathered wood. Conservation principles state that we do not want to harm the historic fabric below the paint layer(s): therefore we do not subject the surface to power washing or to heavy sanding procedures. Sanding is generally done only by hand or light orbital sanders. we then clean and rinse the surfaces and, after drying, apply the coatings. So, are we lying to ourselves? Is the degree of surface preparation sufficient? How do we really tell?

Comment from VCBUD JENKINS, (9/16/2011, 12:21 PM)

David: The Speedheater sounds like a safe way to heat up thermoplastic paint, causing it to bubble and soften so it can be scraped off. This is probably kinder to the sound wood underneath than waterblasting would be, but slower. It would not inject water into the wood, which is good. Beware of heating lead based paint (pre 1978 houses) because the vapors might contain vaporized lead. You are right that the faster the job is done, the less exposed the bare wood gets. Ken: Since you sand the paint off carefully, that is the best way to conserve what wood is left. The very best way to conserve historic wood is to take off board by board and coat it on both sides and all the edges. This keeps water from reaching the wood, making it dimensionally stable. The reason wood falls apart is the taking on of water and then the drying out of the same water, making the wood swell and shrink daily. Using the epoxy consolidants mentioned above will also keep the wood preserved for many years since they penetrate deep into the pores, protecting them from moisture attack. In your sanding, be sure to get the edges, especially the ends of the boards where a lot of water enters. Call me if you want more details.

Comment from Catherine Brooks, (9/16/2011, 2:35 PM)

No worry about lead vapors with safety tested infrared paint remover operating at 400 degrees as low heat. With lead vaporizing at 1000+ and heat guns usually that high, there is the problem.

Comment from Bill Connor Jr., (9/17/2011, 12:00 AM)

Two part Penetrating Epoxy Sealers such as Restore It are quite effective if a bit costly. One scenario where if find these products particularly helpful is the painting off true divided light sash where grey wood is found. Even a small amount of grey wood will prevent adhesion. On one job where we primed and painted without sanding all the weathered wood due to concerns over scratching the glass the coating failed the first winter. I returned and at no cost to the customer removed the failing paint and applied the clear Penetrating Sealer I had no more returns. No primer is required when repainting over this product as it provides good strong adhesion to the new paint.

Comment from David Reynolds, (9/19/2011, 1:17 PM)

@ Bud - thanks for the discussion. @Bill (or/and Bud) - when you speak of a penetrating sealer, do you mean low VOC oil, or water based? (I realize that these terms are getting outdated, but still in wide use.) With current generation products, and given a well prepared surface, does performance differ in gaining and maintaining adhesion without bridging, as Bud describes above? Thanks.

Comment from VCBUD JENKINS, (9/19/2011, 3:41 PM)

Hi David: The penetrating epoxy sealers are low viscosity solvent borne sealers that use capillary action to wick their way down to the solid wood and into the pores. To make them low voc the manufacturer has to use exempt solvents such as PCBTF or Acetone, along with surface tension reducing additives. Here, the danger is flammability, so have a hose ready when painting. Epoxy will give better adhesion than alkyd or acrylic coatings, as long as it is overcoated right away before it completely cures. After a short amount of time the ability of the color coat to adhere to it will drop off, so this is also something to consider. Temperature is also important. On a hot day it might cure too hard to overcoat with good adhesion and on a cold day it might not be cured enough to overcoat because it would still be liquid. If you pay attention to these warnings, the job would be better than just the sanded wood itself.

Comment from Jerry Berggren, (9/22/2011, 12:06 PM)

Some things never change. I heard the same advice more than 20 years ago from a paint manufacturer's representative at a seminar in San Francisco. Dead or decaying wood will not hold paint!

Comment from David Reynolds, (9/23/2011, 11:50 AM)

@Jerry - quite right, wood's a composite, mostly cellulose, lignin, and, in heartwoods especially, some aromatic and resinous stuff. While appropriately formulated epoxies offer exceptional penetration and strength, a surface of sound, entire wood comes first. (That said, epoxies also figure in various patching products and adhesives, but the best technical, economic, and job site solution is what motivated my question. Not sure that I have it yet. Thanks) @Bud - 'Appreciate it. I'm a boatbuilder and somewhat familiar with the various epoxy products for that purpose. Your explanation above helps too. Thanks. The current problem is several thousand sf of quarter sawn cedar and probably pine clapboards and trim on a friend's home in Maine. The current paint failure - maybe at least 10 years old - seems to follow recoating after what must have been a long period of exposure of the siding with only chaulked out, chipped away old paint (lead based too, BTW.) As Bill notes above, 2-part epoxy undercoat is expensive. It would almost certainly be too much so for this job, as would the job site complications. 'Leaves me with penetrating alkyds and current technology acrylics for the undercoat. We'll definitely put in the time to expose sound wood and remove dust (as Jerry points out.) I have photos should that help to clarify, but, at this point, I think that we're not dealing with excessive condensation, unsound wood other than the surface, deluge roof runoff, etc. The now failed latex went on over scraped, but not well abraded, planed, washed, or aggressively vacuumed surface. 'looks like. Again, good article and comments.

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