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Wood Coating Systems
Coating systems, whether they are for wood, steel, aluminum, fiberglass, Ferro cement, or other materials have one thing in common: they are all designed the same way. To build a bridge one does not start by paving the roadway with asphalt and then putting the steel supporting structure underneath. Neither does one start by hanging girders in mid-air and then connecting them to opposite shores. One builds a bridge by starting at both shores and building towards the middle, where the girders connect to each other. Coatings are designed in a similar manner.
At the beginning there is the surface one starts with, and the desired attributes or qualities of the finished surface. Knowing what you want for the finished surface, a supplier can recommend (or you can choose) a chemical type of coating that gives that performance. Sometimes that particular type of coating requires a primer for adhesion. We now have the one or two outer layers defined. Knowing what you have for a starting surface, you can select a product which provides sealing of porosity, protection against corrosion, and intermediate flexibility between a very flexible starting surface and hard outside finish, or vice versa. You do what is needed to the starting surface, working outwards. You design what is needed for the exterior, working inwards. At some point you have a last inner coating which is chemically and mechanically compatible with the first of the necessary outer coatings, at which point the two are joined and you have a coating system.
Wood is relatively porous and often a rough surface, consisting of fibers bonded along their length to other fibers deeper within the bulk of the wood. At the surface parallel to the grain the surface fibers are not as well bonded due to the mechanical forces of cutting the surface, or to weathering. When a pigmented coating is applied to such a surface, the liquid will soak in to some degree, leaving the outer part mostly pigment with less of the liquid (resin) part than the manufacturer originally formulated. The product will therefore not hold up as well as the original product did since the original formula has been altered and degraded.
Wood contains oils, sap and moisture. If one wished to impregnate the wood with a resin system so as to seal it and provide a non-porous surface which is strongly bonded into the bulk of the wood, the sealer should have the ability to dissolve and mingle with those oils, saps and moisture in order to penetrate most effectively. It should not contain pigments or solid fillers as these will clog the pores of the wood and prevent penetration.
Oil based enamel paints or latex paints are commonly used on buildings, but the marine environment is more severe and epoxy or polyurethane coatings are commonly used. Polyurethanes are degraded by contact with moisture before curing so are not suitable for application to bare wood with any significant moisture content. Epoxies are largely sensitive to ultraviolet light, which is a component of sunlight. They will lose their gloss and become brittle with age. The addition of pigments to epoxies can block ultraviolet light from penetrating the epoxy, but will not stop the loss of gloss. Epoxies are relatively insensitive to the presence of moderated amount of moisture. Epoxy systems designed as paints contain about 50% weight pigment and 50% weight resin, with solvents added as needed to make a manageable liquid.
Joints occur in all types of construction where a boat or a building is made form separate pieces. Coatings need to accommodate expansion and contraction of a large surface, and to elongate enough to bridge over joints.
High strength adhesives (mainly epoxies) need to firmly hold two elements in intimate contact and transfer stress from one element directly to another. The less flexible epoxies which make good adhesives do not have the flexibility that makes better coating, while the more flexible (usually latex on buildings, polyurethane on boats) coatings systems have nowhere near the adhesive bond strength to systems designed to be good adhesives.
In house or building applications, a latex acrylic caulk is inexpensive and usually offers adequate performance.
In marine or building applications, under hardware, fittings or between planks (in seams) a material which has good adhesion and moderate elongation is required. The elongation is far more than epoxies or latex caulks can provide, so here urethane or polysulfide sealants are used. Similarly, for sealing cracks in concrete slabs, sealing between a window frame and the wall of a house, or sealing joints or the inner surface of an old wooden rain gutter, the elongations and weather stability of a two-component polysulfide or single-component polyurethane makes it a superior choice as a coating sealant. The reason that all these different material exist is that there are specific needs which each of these materials fulfills. There are some needs which each of these materials cannot fulfill, that is why the other exist.
Every manufacture will claim his product is best for as many applications as he can justify. This is not surprising, as he has a product he is proud of and he is in business to sell products, make money, expand, and provide services which his customers need. In order to best apply these ideas to wood, we need to know more about the nature of wood and the modern epoxy products now used with it.
All wood consists of cellulose fibers bonded together by a natural resin (lignum) which acts as glue. Typically seven to fifteen percent moisture is present (chemically attached to the cellulose) depending on average humidity. There is also a varying amount of "sap", which is a complex mixture of different oils and resins. Hardwoods such as oak have different resins than softwoods such as Fir.
In order to obtain a good bond between wood and any paint or filler, the fibers on the surface of the wood must be strongly bonded to each other and to the filler or coating. The bond must be very flexible since wood expands and contracts as the humidity in the air changes with the weather and the seasons of the year. The surface of wood is microscopically rough, with many surface fibers loosely attached to the bulk of the wood underneath. The liquid portion of paints can soak into the porous wood surface. This can produce a paint with most of the pigment on the surface and most of the paint resin soaked into the wood, and such a resin starved coating will soon fail. Therefore a sealer is important to order to obtain good adhesion between wood and paints.
One of the earliest uses of a primer was as a sealer. Varnish was thinned with mineral spirits and applied to DRY wood. It soaked in and glued the surface fibers of wood to those beneath and stabilized them (to a degree) against expansion and contraction of the wood with changes in humidity and temperature, and kept a topcoat from soaking in and telegraphing the grain of the wood to the surface. This is an example of a sealer primer, also functioning as an adhesion-enhancing primer. There are now modern epoxy-based products that do the job better than varnish, due in part to their higher strength and greater elongation capability of the epoxies.
There is no primer available anywhere, not through specialty suppliers or retail outlets or any other, that functions as an adhesion promoting primer, a porosity-sealing primer, a tannin-blocking primer, an acidity-neutralizing primer and a sanding-primer, (as well as resisting long-term coating failure) all-in-one.
This severely rotted mahogany plywood has been treated with CPES. It's easy to see the epoxy film on the surface, this acts as an incredibility stable surface that literally glues topcoats to it.
The modern use of the word primer has become a non-specific term used broadly and irresponsibly by most paint manufacturers. Literally, a primer was to be the prime coat, or first coat. This derives from the Latin PRIMUS, meaning FIRST. Products now available in the marketplace include adhesion-promoting primers, waterproofing primers used to seal a porous substrate, fillers primers, sanding and surfacing primers, corrosion - inhibiting primers for metal, wood primers intended to block tannin bleed-through of wood topcoats, moisture-diffusion-barrier primers, and probably others. Adding specific adjectives to the word primer conveys a clear concept.
It is not unusual for varnish to peel off wood in six to eighteen months, even when a top-quality varnish is used. Varnish is a highly refined oil-based enamel paint without pigments. Neither varnish, enamel or latex paints are particularly aggressive adhesives, latex least of all.
Varnish neither wets nor adheres well to wood with too much moisture in it, since neither the varnish resins nor the solvents can dissolve water. This is why varnish on some wood makes the wood look lighter, and on some darker. It is not actually wetting the (lighter looking) wood. Wet concrete (in the rain) looks darker than dry concrete. Concrete has porosity, just as does wood. It is an optical effect. The principal is the same on wood. Varnish that has not wetted the wood is not adhering to the wood, which is why it fails sooner.
Epoxy resin systems do not readily dissolve water, but the Clear Penetrating Epoxy Sealer contains a high percentage of alcohol in order to dissolve not only the moisture in woods but also the sap and oils. It is quite one thing for a solvent to dissolve the natural oils in wood (that is easy) but quite another thing for cured epoxy resin system to dissolve the natural oils in wood.
All epoxy products are not the same. Most are made from petrochemicals, and those do not dissolve very much if any of the natural saps or oils in the wood.
This is why that particular resin system in these particular products are made from those very resins that are in the wood itself, so that they can dissolve the natural saps and oils in the wood, this is why is sticks so well to wood.
Because the resin system of this product may be only half-cured a day or two later, we have a very important concept: If we apply something over a surface to which the Clear Penetrating Epoxy Sealer (CPES) was previously applied, but before the CPES is (roughly) half cured, the second material proceeds to cure first, and the resin film of the CPES cures last, gluing down the second material. This is why CPES is such an effective adhesion-promoting primer for varnish. When sufficient CPES is applied to the wood in one application to completely saturate the wood, a thin film of the mixed resins of CPES is left on the surface. The CPES glues down the varnish and the ultraviolet absorbers in the varnish protect the epoxy and the wood from degradation by sunlight. From what our customers tell us the typical life expectancy of a good quality varnish, applied in this manner, is two years or more.
When using clear topcoats such as varnish or polyurethane it is important to know that the ultraviolet absorbers used in all clear coatings are sacrificial. They will eventually die. Therefore, more coats give a longer life. When the ultraviolet absorbers finally "burn out", the ultraviolet from the sun passes through the clear topcoat and attacks both the Clear Penetrating Epoxy Sealer Resin and the underlying wood, breaking down the cellulose fibers. The coating eventually fails as the material beneath it swells and decomposes.
Clear epoxy products are not chemically compatible with ultraviolet absorbers, and so epoxy coatings are not used as clear finish coats. In pigmented coatings the pigments block the ultraviolet. Because the pigments are chemically stable minerals, they do no "burn out" and so this failure mechanism does not exist. Enamel and latex or urethane paints will therefore last longer than clear urethane or varnish coatings, although high quality clear coatings can last several years.
Latex paints, alkyd enamels, urethanes coating and varnish can adhere better and last longer with the Clear Penetrating Epoxy Sealer. Oil-base enamels, oil-base primers or varnishes may be applied the same day or the next, while for most waterborne (latex) paints one should wait two to three days, or more as more of the solvents need to evaporate out of the wood before the waterborne coating can be applied with the best expected results.
Paint formulations are often changed by the manufacturer. Always do a small test with new product combinations to ensure everything works together.
