|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
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
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
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
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
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
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
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
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 www.woodrestoration.com.
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
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
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