Boric acid, or borate salt, is an excellent wood preservative, fungicide (kills rot fungi) and insecticide. It is very low toxicity, has low environmental impact and is relatively cheap. It is an ideal material for treating both hardwood and softwood against attack by fungi (dry rot fungi), and insects. The only real downside is that borate compounds are generally water soluble so must be used in dry environments or protected with a finish of some kind that seals them in the wood.
Borates are available as dry powders (TimBor and others) or as glycol-based liquid concentrates (Bora Care and others). Glycol is intended to improve the penetration of borate into wood fibers but studies have not definitively demonstrated an advantage over simple water solutions. Glycol-based products are also somewhat more expensive than powder forms.
Both TimBor and Bora Care should be applied to dry, unfinished wood surfaces with a pump sprayer or paint roller. Two coats are sometimes needed (see product label). Powder forms like TimBor can also be applied as a dry powder to wall voids for carpenter ant treatments.
Borate wood treatments are generally not available in stores but are available here (DoMyOwn.com, our affiliate).
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