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Essay 4

How to get more life from paint on old, weathered wood

By Steve Smith

Actually, I could have called this article

How to increase your customer satisfaction and your painting business profits

But it's pretty much the same thing. The highest quality paint, the most professional surface preparation, and paint application, don't always give the best results. Paint stores and painting contractors both lose profits and have to deal with unhappy customers occasionally.
The biggest source of failed paint is on old wood, on older buildings, on weathered, slightly deteriorated wood. Even when the old wood looks sound there are all-too-often mysterious failures. Even after cleaning off the loose wood, after fillers and paint it is common to see paint failures only one to three years later.
Why is this? Why is the paint coming loose from the wood?
The wood is actually rotting away from under the paint. This is why paint comes loose from wood.
The old restoration technology consists of cleaning off the decayed areas to where the wood looks sound, applying some filler or caulk to make a smooth surface, and then painting the prepared surface.
In many cases, wood actually rots faster after this kind of repair.
It was not always this way. Long ago, putty was used as a filler, and the putty was made from lead carbonate and linseed oil, and the paint itself contained lead carbonate [white lead] pigment. The lead compounds were mild but effective fungicides, and helped the wood to resist rot by discouraging fungal growth.
Even though the fungal spores [which are everywhere] were on the wood surface, even though the fungi had air, warmth, moisture and a source of food [that's all any spore needs in order to hatch, start eating and grow], they could not flourish because even the most microscopic traces of lead made it impossible for the fungal cells to grow in wood next to paints or fillers containing lead.
About forty to fifty years ago, the use of lead-containing products in architectural paints, fillers or other products was discontinued. Now, there was nothing in any common paint, filler or sealant to stop fungal activity. If any moisture got into wood, rot was inevitable, for air [there is air in wood], warmth and food were all present already.
We all know that if there are water leaks through failed caulking, behind failed flashing, or through a failed paint film, water can get into wood and start the decay process. But what if there are no leaks from the outside? Why does a perfectly sound paint film fail to keep water out of the wood?
Water from external rain may not get under the paint from the outside, but water can still get under the paint.
There is a way that water gets under a painted surface and starts the rot process. Have you ever seen dew [condensation] on some outside surface in the morning? Do you know why it is there?
Everything radiates heat, depending on how hot it is. When there are clouds overhead at night, the surface of the earth radiates about as much heat upwards as the clouds above radiate downwards. Thus, the clouds act as a heat mirror, keeping the planet surface from getting really cold at night. When there are no clouds overhead, the earth radiates its heat away to the night sky, which is at the temperature of the space between stars, about 450 degrees below zero.
When cars, buildings and anything on the surface gets cold enough at night, moisture in the air condenses on those cold surfaces. That is where dew comes from. The temperature at which dew condenses onto a surface is called the dew point. It varies with the humidity of the air.
When you see dew on the outside of a painted surface, you can expect there will be dew on the inside of that same paint film.
Wood contains over a thousand times as much moisture as air. Thus, when the outer building surface is cold enough for condensation to form, the wood just below that painted surface will draw water vapor from the inside and condense it. That wood can become completely waterlogged in a microscopic layer just below the painted surface. Further into the structure, the wood provides enough thermal insulation to keep the temperature above the dew point.
When the sun comes up in the morning, the outer painted surface is warmed, the outside dew evaporates, and a new day begins. Below the paint film, however, there now exists warm, damp wood. These are perfect conditions for fungal spores to hatch, grow, eat the wood, multiply rapidly and teach their young. Fungi need only air, moisture, warmth and food and they now have all four. There are thus a few hours in the morning where the wood rots rapidly, on a microscopic scale. The wood swells when it gets damp, and paint films are more brittle when cold and softer when warm, and may occasionally crack. Eventually outside condensation and rain can enter the wood, keeping it more damp for a longer period. Since water vapor has a volume over a thousand times more than its liquid form, painted wood gets wet rapidly but dries out much more slowly. This gives time for fungal activity and decay of wood under the paint.
Many modern waterborne latex primers do not wet visibly sound, dry wood as well as the older oil-base low-solids primers. Water causes the individual wood fibers to swell, and microscopically separate from each other and the droplets of acrylic resin which are in an emulsion [a suspension of insoluble liquid particles in another liquid]. When the water evaporates the acrylic resin droplets [containing the mineral pigments of the paint] are supposed to coalesce into a film. As the water evaporates, the particles coalesce to some degree on the surface, and eventually the wood fibers dry out, wetted to some degree by the resin film lying on top of them. The fibers dry out and shrink, and the water between them leaves. This results in microscopic air gaps at the primer/wood interface. The [acrylic] resins of latex primers have an utterly different chemistry from that of wood [triglycerides and terpenes] and this also gives less adhesion to the fibers and natural resins of wood. Lacking specific antifungal additives in the latex primers [some may, others may not], this change in product formulations by manufacturers gives a greater tendency in recent years for rot to develop under the dried paint film.
There has been an increased use of wood grown more rapidly, within the last half of the twentieth century. This is visible by its growth rings spaced an eighth to a quarter-inch apart, where high-quality wood has its growth rings less than a sixteenth of an inch apart. Faster-grown wood material has more porosity and thus less resistance to water absorption into the wood and this factor also plays a large role in premature paint failure in the last few decades.
It is therefore not unusual for old, painted wood surfaces to show paint failure in one to three years, and this reflects badly on the paint store, the paint manufacturer and the painting contractor, all of whom are doing the best they can with their existing resources.
Thirty years ago the material known as Lignu Impregnating Resin was invented by Steve Smith. Today, this product has a long history of helping wood resist the common form of deterioration under a paint film. This is done without the need for lead or any pesticide. The natural properties of wood and its resins are used in a novel way to discourage the start-up and continuation of fungal activity under a paint film. The Lignu Impregnating Resin keeps the paint glued to the wood and thus helps it resist the microscopic fungal activity that causes paint failure.
The Lignu resin system is made largely from the natural resins of wood itself. This was done so that, when it was finally cured, the impregnated and microscopically restored wood surface would have toughness and flexibility comparable to the original wood. This eliminates the failures so commonly seen when a stiff resin coating or hard filler is used to "repair" wood. The proof of treated and untreated wood having similar mechanical properties, and the impregnated wood stronger, are published in a scientific paper at
Since there is naturally a high percentage of water in wood, normally eight to twelve percent but, when abnormally wet, often twenty to thirty percent by weight, the Lignu Impregnating Resin was formulated to dissolve much of this excess water. This allows it to wet and penetrate wood far better than any other product. Many studies have been done which shows this superior penetration.
The selective penetration into the deteriorated parts of wood is shown in the time-lapse photographs at the wood restoration web site. These show that even in apparently sound wood there are microscopic channels of abnormal porosity. These were invisible until discovered by the research staff of Smith & Co. while preparing the wood restoration paper. No one knew that the fungi had eaten microscopic channels into the wood, along single wood fibers, farther than anyone could see. This is why the failure of paint on old but apparently sound wood had remained a mystery all these years, since lead was removed from exterior paints.
The Lignu Impregnating Resin rapidly absorbs into the natural porosity of low-quality wood, as well as deteriorated areas of wood. After the solvent content has evaporated, the impregnated wood has porosity comparable to natural wood, thus allowing the wood to "breathe", meaning allowing the normal movement of water and water vapor within wood. This is important, because the accumulation of excess amounts of liquid water anywhere in wood promotes rot. Fungi do not flourish when the moisture content of wood is below about ten to fifteen percent. The normal moisture content of wood protected from the weather is about eight to twelve percent. Fungi flourish when the moisture content of wood is above roughly twenty-five percent. A small amount of water on wood in a small region will not immediately promote rot, as the water migrates into the greater volume of the wood and some excess evaporates, and the natural ability of wood to pass water internally maintains the overall moisture content in a safe range. Water does not migrate rapidly enough to prevent rot under a paint film, as less than an hour of warmth is enough to allow a little more decay each morning.
The film of Lignu Impregnating Resin under the paint discourages rot in another way; it is hydrophobic, thus discouraging the condensation of liquid water in a treated area behind a paint film or where our epoxy filler has been used to restore a deteriorated space.
The resin system of Lignu Impregnating Resin takes days to cure, and so it acts as epoxy glue to chemically bond the paint film to the wood. This eliminates the microscopic air gap among the fibers behind a paint film on wood, and replaces it with a flexible glue that bonds the paint to those fibers and further bonds those fibers into the sound wood.
The wood impregnated with the wood-derived Lignu resin is no longer plain cellulose, and is not easy for fungi to digest, and tastes bad to such small life.
In this manner moisture in wood is reduced, air is reduced, and the available food supply is made less attractive. By removing these key elements necessary for fungal growth, and by actually gluing the paint to the wood, the early failure of painted wood is reduced to negligible levels and many more years of paint life are commonly seen.

Copyright © 2002 Steve Smith All rights reserved

Essay 1
An introduction to paint, varnish and the Lignu Resin on wood
Essay 2
Clear coatings on wood
Essay 3
Essay 4
How to get more life from paint on old, weathered wood
Essay 5
Essay 6
Essay 7
Essay 8
What's the Matter?

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