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Skandinavisk Impregneringsservice

Essay 2

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 to gray.
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 joint.
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 tests.

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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