Borate products compatibility with
Smith's Clear Penetrating Epoxy Sealer™


When wood is required to have rot-resistance superior to the natural rot-resistance of the wood itself, borate compounds are usually the first choice. However, painting over some kinds of borate treatments can cause problems later, including early or total failure of the paint. We begin with a discussion of the common types of borate treatments, followed by a discussion of how they might be used to improve longevity of both the wood and the paint, in which Smith's Clear Penetrating Epoxy Sealer is commonly used to advantage to attain both goals.

The residual fungicidal resistance of wood treated with borate compounds is well-known. There are generally three kinds of borate-treatment products. One involves solid borate rods inserted in drilled holes in the wood. The second involves treatment with a liquid solution of disodium octaborate and ethylene glycol (the automotive antifreeze of decades past), and the third involves treatment with a water solution of only disodium octaborate itself, a highly water-soluble borate salt, much more-so than the other borate salts or boric acid. Each of these three treatment methods has its advantages and disadvantages, and each has varying degrees of compatibility with Smith's Clear Penetrating Epoxy Sealer.

Solid borate rods

Solid borate rods may be effective in that they slowly dissolve in the natural moisture of the wood, and migrate by diffusion throughout each single wood element. However, this migration is very slow, and may take many months or longer to diffuse throughout the wood. There is no added water, so the wood element remains at its initial moisture content and other painting operations may be done immediately. It is, however, destructive of the wood element, and in historic restorations this may not be justifiable.

Water solution of borate salt

Treatment with a water solution of disodium octaborate (typically a 5% solution by weight) is effective, and a known amount of borate can be applied to the wood. The absorption of the water solution can be determined in a quantitative way by weighing before and after treatment. Thus, a specified level of borate-content of wood element can be done in a deterministic manner. But, the wood now has a moisture-content too high for immediate painting operations, and the wood must be air-dried (forced convection is preferable, for production efficiency) to the ambient-equilibrium moisture content depending on geographic location (typically 8-12%). This may take several days to several weeks, depending on ventilation and geometry.

Borate-glycol solution

Treatment with a borate-glycol solution may appear advantageous, for ethylene glycol is also mildly fungicidal, but such products have two disadvantages:

First, being highly water-soluble and, indeed hygroscopic(attracts water from the air), it can increase the water-absorption of the paint, and certainly the water-content of the wood immediately under the paint, and these factors reduce the ability of the paint to remain bonded to the cellulose fibers at the surface of the wood. In short, it can lead to premature paint failure, although this is difficult to predict specifically because there are so many different paint formulations and wood-treatment levels will vary depending on wood species, age and extent of deterioration if any.

Second, ethylene glycol is volatile, evaporating out of an unpainted wood surface in a matter of a very few months or even less, depending on ambient temperature and air circulation; its vapor pressure is one ten-thousandth of an atmosphere, a small but by no means insignificant amount. In confined spaces the vapor will accumulate to levels that bring one to question health issues, for it is known be toxic to humans and animals. Being volatile, it is considered a Volatile Organic Compound (VOC), and in most locations there is an Air Quality Management District that regulates such things. It does, however, evaporate sufficiently slowly that it may be painted-over (some paints are not compatible), whether on-site or in a shop, and the wood architectural elements reassembled and installed, as the case may be. There now arise both evaporation and paint-compatibility issues, for the ethylene glycol WILL diffuse through any paint overcoat.

There is another liability associated with borate-glycol products in that, depending on the architectural circumstances and coating-schedule, some of the ethylene glycol may evaporate on the inside of a building. While there may be no known health-issue traceable-with-certainty to that small amount of ethylene glycol vapor over a period of time, a Predatory Plaintiff's Lawyer can always bring the issue to a legal action, causing people far-removed from a cause-of-action to spend inordinate sums on defense attorneys.

Summary of borate treatments

Borate salts in general are not toxic to humans or mammals, and are natural micronutrients for plants. They are, however, relatively toxic to most fungi, as well as to certain insects that have an exoskeleton (shell on the outside instead of bones-on-the-inside). Thus, a simple water-solution of a borate salt (the most soluble is disodium octaborate) is a safe and effective method of preservative-treatment of wood, and has no long-term risks to human or animal health or of lawyer-attacks.

Smith's Clear Penetrating Epoxy Sealer and its compatibility

Treatment of wood with Smith's Clear Penetrating Epoxy Sealer renders it resistant to passage of liquid water, but it remains highly permeable to water vapor. Thus, the wood is said to be able to "breathe". Since borate salts are water-soluble but cannot migrate through any resin system, treatment first with a borate salt and second with Smith's Clear Penetrating Epoxy Sealer renders the borate salts trapped inside the wood, unable to diffuse out by contact with other wood elements and unable to be washed-away by exposure of unpainted surfaces to rain or condensation.

Since Smith's Clear Penetrating Epoxy Sealer is a highly effective adhesion-promoting primer, a subsequent application of any paint or coating system can give a long-lasting durable finish, protecting an effectively-treated wood element. Depending on the method of treatment, however, there are issues to be addressed.

Borate rod issues

If the wood element had holes drilled and then the epoxy treatment and then the borate rod inserted, that borate will not be effective in protecting the wood because the borate will be unable to diffuse into the wood since it is in contact with only epoxy-treated wood, which will not allow passage of the borate salt. If the rod were inserted and the hole sealed prior to epoxy treatment, then the rod would be effective in protecting the wood, provided that there was not a crack that allowed the epoxy to enter the borate-wood-cavity. This gives an uncertainty of effectiveness, since one is not going to cut apart the wood element to see if the borate-rod-cavity is free of epoxy intrusion.

Borate-glycol issues

When a borate-glycol solution is absorbed into wood, application of any epoxy product, solvent-borne or not, on a surface saturated with ethylene glycol results in absorption of the glycol into the epoxy-resin system. This degrades adhesion of any epoxy to wood. Ethylene glycol will actually absorb into even a cured epoxy system and promote excessive water-take-up by the cured epoxy resin. This reduces the adhesion-effectiveness of any epoxy product to anything else in a wood-epoxy-paint-glue system. With time, months or years, the glycol will eventually evaporate, but the adhesion-loss is irreparable, and does not recover when the glycol leaves.

Many latex paints and oil-base paints are adversely affected by long-term exposure to ethylene glycol, and they may not even properly develop adhesion when first applied. Drying and/or curing are adversely affected. Water-emulsion (latex) paints will not be able to evaporate fully their water and thus coalesce properly, for the absorbed ethylene glycol will hold excess water in the (drying) paint, thus preventing the resin-phase of the latex paint from forming a film ("coalescing") as it is designed to. The net result is that latex paints may fail prematurely when first rained upon, or may remain sticky, or exhibit other aberrations. Such paints are said to be incompatible with such a wood treatment.

Borate/water-solution issues

When the borate-water solution has been applied to the wood, whether by spray or immersion, the wood will be wet. Wet wood is not to be immediately painted. It is to be dried, ideally until the moisture-content of the wood is in equilibrium with the local humidity of the air. Some paints may not be compatible with such a treated-wood surface due to the alkaline nature of the borate salt. Application of Smith's Clear Penetrating Epoxy Sealer to the wood surface after it has dried seals the borate salts away from the surface, and then latex paints or solvent-borne paints may be applied on top without compatibility issues. In over forty years Smith & Co. has not found a primer or topcoat that is incompatible.


The preservative benefits to wood of a borate treatment, and the benefits of Smith's Clear Penetrating Epoxy Sealer (also known as MultiWoodPrime) in promoting paint adhesion may both be obtained without paint-compatibility issues, by using the appropriate type ofborate-based preservative.

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