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Community and Q&A

Home-brew wood preservative

Martin Holladay | Posted in Green Building Techniques on

I recently received an e-mailed question from a friend who is building a house in Costa Rica. The topic is beyond my technical expertise, so I’m turning to the GBA community in hopes that a chemist can provide useful advice.

Here’s what he wrote:

“We are about to treat the wood for our ceiling in the house we are building in Costa Rica. The wood is what they call here Laurel — Cordia alliodora. It has no particular resistance to insects, as teak does. The wood should stay dry and not be exposed to rain or splash.

“We were going to use Xylibor wood preservative, but it has hexavalent chromium, which I really want to avoid. So I searched the web and found a lot of copies of home-brew recipies for Timbor and Boracare. We made a batch of the Boracare copy using borax, boric acid, and propylene glycol, but heating propylene Glycol to 260 degrees F is scary (hotter than flash point), and the high concentration of glycol discolored the wood and left a blotchy look. Also the high borate concentration left a coating of white dust.

“So, we switched to making the home-brew Timbor. We made both the 10% and the 15% versions. Both worked fine, dried easily, and left no white coat of borate crystals. I also tested adding a tablespoon of propylene glycol to a cup of the 10% solution, and 2 tablespoons to 1 cup of the 15% solution. The added glycol slowed the drying noticibly, but not beyond what is practical for us.

“Is there any problem with having glycol in wood under a clear acrylic finish (tekwood)?

“What is the purpose in the home brew Boracare recipie of heating the mix to remove the water of crystalization?

“Why do both these mixtures need to be used quickly?

“Is this home brew 15% Timbor solution effective against termites and powder post beetle?

“Does it make sense to add the propylene glycol for greater penetration and for its anti-fungal and insecticide properties?

“We also plan to add Dragnet, a permethrin product.

“Thank you for any light you can shed on this topic.”

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  1. GBA Editor
    Martin Holladay | | #1

    Any GBA readers want to provide advice here?

  2. Expert Member
    Michael Maines | | #2

    Martin, I'm no chemist but it seems like many of your questions are anwered here: The instructions say heating the solution makes it stable; it's when it's mixed with water just before application that it must be used quickly. I imagine it might have to do with water neutralizing the acidic mix but I'm not sure.

    The glycol compound is a surfactant. There are differences between glycol compounds (polyethylene, propylene, etc.) but probably not enough to matter for this application. Woodturners often use PEG (polyethylene glycol) to displace the water in green blanks. More info here: If you are applying the solution to dried wood there may not be a reason to use the glycol compound, and apparently it can leach out in warm weather, so I'd be inclined to avoid it if possible.

  3. GBA Editor
    Martin Holladay | | #3

    Thanks very much for your helpful information. Much appreciated.

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