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Painting Borate Treated Wood

The treatment of wood with Borate is a common means of offering rot resistance to the wood. However, painting borate treated wood can cause significant problems later, including early or total failure of the paint. If painting borate treated wood is required, the means of treatment and the mechanism of painting should be addressed at the same time.

Why Do We Treat With Borate

Borate salts in general are not toxic to humans or mammals, and are natural micronutrients for plants.  They are, however, toxic in varying degrees to most species of fungi, as well as to certain insects that have an exoskeleton (shell on the outside instead of bones-on-the-inside).  Thus borate salts (the most soluble is disodium octaborate) represent a safe and effective method of preservative-treatment of wood, which has no long-term risks to human or animal health or of lawyer-attacks.This article presents the various means of getting borate compounds into wood, the problems that each method can present when painting and the solution to ensure successful adhesion of the paint.

How do we treat wood with Borate

Three common methods exist for the treatment of wood with Borate compounds. Each brings its own problems and requires different approaches if the wood is to be successfully painted afterwards.The four common mechanisms of treatment are:

  • Water solution of Borate Salt. An aqueous (dissolved in water) solution of Disodium Octoborate which is painted on to the wood surface.
  • Solid Borate Rods or Pellets. These are inserted into small holes drilled into the wood.
  • Borate Glycol solution. A liquid solution of Disodium Octoborate and Ethylene Glycol which is painted onto the wood surface.
  • Zinc Borate Treatment. Since Zinc Borate is insoluble in water a solution is painted onto the wood which deposits Zinc Borate within the wood itself.

Water solution of borate salt

painting borate treated wood sodium octaborate

Treatment with a water solution of disodium octoborate (typically a 5% solution by weight) is promptly 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.  It has a disadvantage in that 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.

Painting Aqueous Borate treated wood

When the water-borate 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, so compatibility must be confirmed first.

Epoxy treatments and Aqueous borate treated wood

Treatment of wood with Smith’s Clear Penetrating Epoxy Sealer renders it resistant to the passage of liquid water whilst leaving the wood highly permeable to water vapour.  The wood is said to be able to “breathe”, and breathable paints may still be applied to good effect.  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 porosity-sealing and 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.  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 of the sealed surface without compatibility issues.  In over forty years Smith & Co. has not found a primer or topcoat that is incompatible.

Solid Borate Rods

The solid borate rods are 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 slowly 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.

painting borate treated wood rods

Painting with solid borate rods

Some paints may not be compatible with the alkaline nature of the borate salt, so compatibility must be confirmed first, as the borate will migrate through the timber.

Epoxy treatments and solid borate rod treated wood

There are no possible chemical incompatibilities between the borate rods and a post applied epoxy resin system. The borate rods must be inserted and sealed into the wood before any epoxy resin treatment is applied however, as the borate will be unable to diffuse through the epoxy resin system. The seal must be complete as any ingress of resin into the cavity containing the rods will seal the borate into the rod. Provided these provisos are maintained, the same benefits of increased paint adhesion, guaranteed paint compatibility and a breathable sealed surface as described for the aqueous borate solution are available by post treatment with Smiths Clear Penetrating Epoxy Resin.

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, 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 vapour pressure is one ten-thousandth of an atmosphere, a small but by no means insignificant amount.  In confined spaces the vapour 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.There is another potential 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 vapour 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 defence attorneys.

Painting Borate glycol treated wood

Ethylene glycol evaporates sufficiently slowly that it may be painted-over (some paints are not compatible with it, so check first), whether on-site or in a shop.  There now arise both evaporation and paint-compatibility issues, for the ethylene glycol WILL diffuse through any paint overcoat.  Being highly water-soluble and, indeed hygroscopic (it 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. These factors reduce the ability of the paint to remain bonded to the cellulose fibres 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
  • Wood-treatment levels will vary depending on wood species, age and extent of deterioration

Furthermore 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 set, for the absorbed ethylene glycol will hold excess water in the (drying) paint. This prevents the resin-phase of the latex paint from properly forming a film as it is designed to. The net result is that latex paints may fail prematurely when first rained upon, may remain sticky or exhibit other aberrations.  Such paints are said to be incompatible with such a wood treatment.

Epoxy Treatments and Borate glycol treated wood

Epoxy treatments cannot be used to offer a surface suitable to ensure paint adhesion on Borate glycol treated wood for a variety of reasons:

  1. As a borate-glycol solution is absorbed into the 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.
  2. 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.

Zinc Borate

The final mechanism that can be used to treat wood with borate is to use Zinc Borate. Zinc Borate is benign in timber, and will not migrate out as it is almost insoluble in water. The problem with it’s lack of solubility is that the Zinc Borate has to be created in the timber itself, requiring the use of a two part preparation.  Whilst this is more expensive than the other Borate treatments, it’s use is readily justified for valuable, historic or otherwise exotic applications, and the wood may be stained, overpainted or otherwise treated after application.