Sash Preservative Treatment...

Repair methods, procedures and details.
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Sash Preservative Treatment...

Postby johnleeke » Tue Jun 15, 2021 5:16 pm

I often apply a migrating borate preservative (like BoraCare) to treat the inner volume of the wood, and then apply a penetrating water-repellent preservative to the outer weathering surfaces. These treatments are also effective for window frames, sills and casings that are exposed to the weather.

Borate preservatives are highly effective and easy to use. I treat many of the sash I work on by drilling holes in the bottom rails and stiles and filling them with undiluted BoraCare concentrate or Jecta paste form borate preservative. For the stiles I drill in the edge of the stile, two or three inches up from the lower joint. For the lower rails I drill in the glazing rabbet, which will be sealed with putty and paint. If I have the lower joints apart, I apply concentrate on the surfaces of the mortises & tenons before re-assembly. If the sash will be installed in a damp environment I will treat with diluted BoraCare.

Bora-Care has to be applied to the wood at a certain rate to be effective at preventing fungus. The rate is 1 fluid oz. of Bora-Care concentrate to 6 board feet of wood (I think those numbers are right, check the instructions to be sure). Note that board feet is a volume measurement, not a surface measurement. This is important because the Bora-Care will migrate throughout the entire volume of the piece of wood, even if you are just spreading it on the surface.

This rate of application is different from (and not to be confused with) the mixing ratio.

The mixing ratio depends on how you are going to apply the Bora-Care. When I apply it via holes drilled in sash stiles and rails, and sills, I mix the Bora-Care 1:1 with water. This diluted solution will usually last for 1 to 2 days in the application bottle, before it begins crystallize. (by the way, never pour any Bora-Care concentrate or dilution back into your main supply container of concentrate because it may kick off a total crystallization of the concentrate.) I have used this same mixing ratio for applying Bora-Care to the surface of the back sides of jamb and header boards and sills. A different mixing ratio may be needed if applying to surfaces that will be painted. (I don't know what it is because I never do that.)

So, this is how I do a BoraCare preservative treatment:

1. Measure the dimensions of the wood to be treated and calculate its volume.
2. Determine how much concentrate is needed to treat that volume of wood at the rate of 1 fluid oz. of Bora-Care concentrate to 6 board feet of wood.
3. Measure out that amount of concentrate into an application container, and add an equal amount of water to get a 1:1 mix ratio, and then mix the dilution.
4. Apply the dilution to the wood.

[size=150]Application of Borates to Window Sash[/size]

I sometimes treat sash with borates, especially if there are wood-epoxy repairs, or splice glue joints that would prevent the stiles and rails from drying out via the end-grain or side edges of the sash. I always use a migrating type of borate preservative and apply it in the stiles via 2 or 3 diagonal holes drilled in the side edge, and in the bottom rails via 2 or 3 holes drilled in the glazing rabbet, unless the sash is apart, when I might apply the borate on the tenon and inside the mortise before reassembly.
The amount of borate to apply (for liquid or rods) is based on the volume of the wood treated, the manufacturer's directions say how much per volume, but more is not better. Too little and it does not prevent decay and bugs. Too much can cause paint adhesion and hardware corrosion problems.

With liquid application of borates the holes can be much smaller (typically 3/16" to 1/4" diameter) and weaken the sash much less than with holes that are big enough for solid rods (typically 5/16" to 3/8").

[size=150]Resources for Borate Preservatives[/size]

[b]Product Types, specific product names:[/b]

Liquid form: Liquid waterborne borates soak immediately into the wood and begin protecting it right away.
Products: Boracare

Solid rod and pellet form: Borate rods can take longer to actually begin protecting the inner volume of wood because there has to be enough water to dissolve the solid form of the salts.
Products: Bor8 Rods

Dry powder: A finely divided power that can be sprinkled on the surface of the wood or disolved in water and applied on the surface or in holes. Timbor is another borate-based wood preservative that is very different from Boracare. Timbor, when mixed with water, will soak slightly into the surface of the wood and then just remain there on and near the surface. Boracare will soak into the surface, and then over time it will migrate throughout the entire inner volume of the piece of wood. In my work that's a valuable characteristic that makes it worth every cent. I usually pay about $90 for a gallon. One gallon is enough to treat 768 board feet of wood, so that's only 12 cents per board foot--such a deal. Both are equally potent at protected the treated wood when applied at the same rate, but it's very difficult to treat more than the outer skin of the wood with Timbor.
Products: Timbor


Wood Care Systems

ConServ Epoxy LLC


Ohio State University Extension Fact Sheet on wood rot: ... ticles.php

Study on the adhesion of epoxy to borate treated wood:

Check Save America's Windows
for the Preservative Treatments topic, where I list the Rules of Thumb for the use of preservative in window work. These rules will help you avoid preservative-coating incompatibilities.

[size=150]Question & Answer[/size]

[quote]1) Should it be painted over the entire bare wooden sash?[/quote]

I hesitate doing this because a film of salt crystals may be left on the surface and could affect paint adhesion. I have not done comparative testing to know if this is actually a problem. West System has done some testing that shows that borate preservatives can affect adhesion of epoxies in a negative way.

[quote]2) Should it be applied to open end-grain for maximum penetration?[/quote]

Penetration will be faster and deeper at end-grain compared to side grain, but over the long-term the borates will migrate throughout the volume of the wood and eventually settled into a state of equal distribution throughout the volume of the wood.

[quote]3) Would allowing edges(rails & stiles) to soak in it for a few minutes be all that is necessary? (application to susceptible areas)[/quote]

In the short- and near-term (days and weeks) application to susceptible areas gives some immediate protection, but over the long-term (several months) the borates settle into a state of equal distribution throughout the volume of the wood. This is why applying enough to treat the entire volume of the wooden part is particularly important.

[quote]4) Will applying to entire sash have any adverse compatibility with other materials such as the putty, linseed oil, paint etc?[/quote]

Applying to surfaces may affect the adhesion of other materials, unless there is enough time for the borate salts to migrate into the wood, which could take, days, weeks or months. The drier and the more dense the wood the slower the migration.

[quote]5) From my research, borates contains salt. On later sash where metal pins are used, could this cause corrosion? How about corrosion of the glazing points? In some older sash hand wrought nails were used, would the salt have any effect?[/quote]

I suspect the concentrations of borate salts that effectively protect the wood from fungus and insects is low enough that the less reactive metals would not be significantly corroded. I have not seen any evidence of corrosion that I could attribute to borates, although I have not done comparative testing.

Since you are applying treatments to prevent decay of the wood it is good to have a clear understanding of how and why wood decays. See pages 5-7 of the of the Wood-Epoxy Repairs report
for the Understanding Wood decay topic.

[quote]Is the use of Boracare compatible with pre-treatments like Water Repellent Preservatives like the Forest Products Laboratory's recipe or oil-consolidators like penetrol? If yes, which goes first, the Boracare or the Penetrol? [/quote]

A Boracare treatment can be applied to the inner volume of the sash parts through drilled holes either before or after surface treatments like Penetrol or the FPL recipe. I often apply Boracare after the surface treatments, just before glazing.

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