Clear coatings on wood
We can understand how clear coatings on wood can last for
years when exposed to the elements, by learning how and why
coatings fail. The "why" is three-fold: Water attacks
the coating or the wood, ultraviolet light (which is an invisible
part of both natural sunlight and interior fluorescent light)
does the same, and the wood substrate moves.
These causes bring about different effects. The most obvious
are yellowing, loss of gloss, tearing of the coating, cracking
and finally flaking of the coating. Somewhere along the line
the wood loses its originally attractive color and bleaches
Some of the above result from loss of flexibility, and this
will manifest as a cracking, tearing, or peeling of the film.
The main reason is degradation by ultraviolet light, which
slowly breaks molecular chains in the coating [a polymer made
of many molecules intertwined and connected]. When this happens
the molecular fragments (called "free radicals",
more about those later) will glue themselves onto neighboring
polymer chains, making extra cross-links. These are extra
branches in a chain, like rungs on a ladder. As more cross-links
are made, the coating loses its elongation capability. That
is to say, it becomes stiffer and cannot stretch as much as
the natural expansion of the wood, and eventually cracks and
tears and flakes. Polyurethanes, traditional varnish and,
for that matter, any clear finish will get more brittle with
age. The reason old, flaking varnish curls outwards is that
the outer surface becomes shorter than the inner surface due
to the extra surface cross-linking from the ultraviolet light.
There are special chemicals designed to trap and neutralize
these free radicals before they can do their damage. They
are called antioxidants (something like vitamin-E, actually)
and they work the same way your antioxidant vitamins work
to keep you healthy.
Conventional varnishes cure by a chemical reaction between
the oil and the oxygen in the air. This is called oxidation,
and the addition of antioxidants to a conventional varnish
would poison the curing reaction. It is therefore impossible
to add antioxidants to varnish and thus any varnish will lose
its flexibility fairly rapidly with exposure to the sun.
Where the coating was applied to two adjoining pieces of wood
and bridged over them, relative motion may tear the film loose
from the substrate without the film itself failing. The visual
result of this is usually a whitish line appearing in the
clear coating over the wood joint. As the coating lost its
flexibility, it became stiffer, and stretched only with more
difficulty. Eventually the force required to stretch the coating
over the joint exceeded the shear strength of the wood or
the peel strength of the coating's adhesive bond to the wood,
and the coating tore loose some amount either side of the
Water causes a loss of film strength
.it will tear more
easily, and stretch less before it fails. The reasons are
technical, and have to do with chemical reactions between
water and some kinds of plastics, acrylics and others, which
lead to decomposition of the material. Some urethanes, some
epoxides, and the reaction products of certain natural oils
(such as linseed oil or tung oil) or other kinds of resins
called alkyds are more resistant to water than some other
materials such as acrylic resins or some polyester resins.
Fiberglass boats develop gel-coat blisters which are a result
of water attacking that polyester resin.
Water may cause a chemical decomposition or swelling of the
wood beneath the coating, allowing the bond between the wood
and the coating to fail. Ultraviolet light also causes chemical
decomposition of wood.
A chain is only as strong as its weakest link, and so it
is necessary to not only improve the varnish or other clear
coating to obtain a longer life, but to improve the stability
of the wood surface. This gives any topcoat something better
to stick to.
There are many different definitions of the word "primer",
depending on the specific functions being performed. Manufacturers
of clear coatings usually design some sealer-primer, adhesion-enhancing
primer or wood-stabilizing primer for use with their coating
products. The oldest and simplest of these, used with any
varnish, was to thin the first coat of varnish with mineral
spirits and allow it to soak into the wood. This is commonly
done today. It does not develop a high-strength flexible bond
to the wood, because varnish is not that flexible. It does
not wet the wood very well, because wood contains water and
the solution of varnish in mineral spirits is very water-repellent.
For thirty years, Smith & Company has manufactured a primer
for wood which is compatible with alkyd or latex paints or
clear coatings or varnishes. Today, in Europe, it is called
Lignu® Impregnating Resin. It dissolves the natural moisture
of wood while it impregnates the wood substrate with a water-repellent
resin system made largely from the natural resins of wood
itself, and bonds the wood surface fibers together and into
the wood substrate where there was open porosity. This gives
a stronger surface, better attached to the bulk of the wood
itself, and thus creates better water resistance of the wood
substrate as well as better topcoat adhesion. It bonds the
coating to the wood with a tough, flexible adhesive, and this
bond is stronger than the bond of varnish or other finishes
directly to bare wood. Thus, the sealer glues down the varnish,
while the ultraviolet absorbers in the varnish protect both
wood and sealer from the sunlight.
Wood consists of hollow fibers of cellulose (a kind of sugar,
very tasty to fungus and termites) glued together by a material
called lignin. Lignin is a very hard, strong resin (chemically
similar to the glue used to make plywood) which is very resistant
to water, but is decomposed very quickly by ultraviolet light.
Ultraviolet light attacks almost everything. All organic compounds,
whether synthetic or natural, will eventually be attacked
and broken down by ultraviolet light. Even some of the best
urethane paints will lose about half their gloss in two years
of outdoor exposure. It is not enough to make a clear coating
which is not much degraded by ultraviolet light, as such a
coating would simply transmit the ultraviolet light through
to the wood underneath.
Therefore, ultraviolet absorbers were invented. The most effective
are chemical compounds, which act as magnets for ultraviolet
light. When a molecule of this absorber material captures
a photon (light comes in small units; they are called photons)
it converts the energy of the ultraviolet photon into heat.
When it does this, the molecule vibrates. The phenomenon is
very much like ringing a bell. We know that if you strike
a bell often enough, the bell will crack from metal fatigue.
The molecules of ultraviolet absorber wear out in the same
way. Eventually they will die and no longer absorb ultraviolet
light. The more ultraviolet absorbers the manufacturer puts
in the clear coatings, the longer the coatings will last,
assuming that high-quality ingredients are used and the coating
itself is correctly designed.
Clear coatings containing ultraviolet absorbers must be applied
to some minimum predictable film thickness, so that there
is enough ultraviolet absorber chemicals over the wood to
afford enough protection to the wood to obtain good life and
color stability for the wood. When the ultraviolet absorbers
burn out, the wood will lose its color, becoming gray. For
a high-quality varnish, this is about one quart or one liter
for every 25 square feet or 2.5 square meters.
Besides absorbers, another kind of ultraviolet protection
is small particles of some minerals. They are small enough
to pass most visible light but big enough to scatter and reflect
most of the shorter-wavelength ultraviolet light. They never
burn out, but they have the disadvantage that as one adds
more or makes a thicker film, there is a noticeable haziness
or blurring of the wood underneath.
Sometimes it is desirable to stain wood before a clear coating
is applied. In general, waterborne stains are compatible with
waterborne clear finishes, whereas solvent-borne stains must
be used with solvent-borne topcoats. The reason is that the
solvent-borne finishes tend to dissolve the resins of waterborne
products, and not to stick well to them.
There is a tremendous difference between a varnish or any
other clear finish from one company or another, or even between
similar products from the same manufacturer. Maintenance of
gloss is a property of only the topcoat, and you should choose
your varnish based on reputation and independent outdoor ageing
Copyright © 2002 Steve Smith All rights reserved