Chemotherapy for Wood Rot

Once rot gets a toehold in wood, it is difficult to cure completely, like a cancer.

Digging out the rotted wood will still leave spores and water in the sound wood. After these

cavities, produced by the digging, have been filled with material such as epoxy, rot will continue to flourish underneath. Some products purported to make rotten wood sound and prevent future rotting, penetrate only until they meet water. Generally, under the repair, the rotting continues

Commercial Products sold as a treatment for dry wood to prevent further rot are most ineffective against established rot in wet wood. There are two inexpensive and commonly available materials which will kill rot in wood. and prevent its recurrence. First, there are the Borates (Borax, Boric acid mixtures,) which have demonstrated to prevent new wood from rotting and killing rot organisms and wood-destroying insects in infested wood. Second, there is ethylene glycol, most readily available as automotive antifreeze/coolant, For killing rot spores and wood-destroying organisms, nothing works better than ethylene-glycol antifreeze, unless it is antifreeze mixed with borates. Glycol is toxic to a broad spectrum of organisms, from staphylococcus bacteria to mammals.

Both borate solutions and glycol penetrate dry and wet wood as well because they are water-soluble; penetration by glycol is especially assisted by its extreme hygroscopicity (its strong attraction to water.) Neither of these is a permanent solution solution to rot in wood which is constantly exposed to water because below waterline, they eventually will be extracted (dissolved) out. Glycol was first perceived as a wood stabilizing agent, rather than a fungicide or rot preventative.

There are two types of borate products commercially available for wood treatment: solid sodium octaborate for making solutions in water (Tim-Bor and Ship-Bor) and a 40% solution of sodium octaborate in ethyl glycol (Bor-Care.) Equivalents and more concentrated solutions can be easily prepared from Borax, boric acid and antifreeze at a much lower cost.
Glycol has an advantage over solutions of borates in either water or glycol. Glycol penetrated rapidly through paint, varnish and oil finishes (except epoxy and polyurethanes) without lifting or damaging those finishes in any way. The dyes in glycol antifreeze are so weak, they do not discolor even white woods such as pine. Once bare wood treated with glycol or borate solutions dries, it can be finished or glued. When a borate solution leaves a white residue, the surface should be washed with water and allowed to dry again.

A preferred method to treat rot might be to soak the soft wood, or evidence of rot, in antifreeze, even if you can’t do any other repairs at the moment. Paint it on or stray it on with a coarse spray. Avoid mist-like spraying to avoid breathing in unhealthy amounts of glycol.
Glycol should be used lavishly on suspect wood, which may readily absorb 10%-20% of its weight in antifreeze. Surfaces well away from damaged wood should also be coated. Extract weak, rotted wood and add additional glycol to thoroughly wet exposed wood. Add a 25% borate solution and allow to soak for several hours. Fill void with epoxy putty or a piece of sound treated wood as required.

Make your own home-brewed borate wood preservatives by using the formals listed below and you will produce a borate wood treatment which will work as well or better than commercially available products.

Home-Brew Water Solution of Borates:
Based on U.S. Navy specs, a ration of 60% borax and 40% boric acid gives the maximum solubility of borates in water. A mixture of 65% water, 20% borax and 15% boric acid (by weight) will yield a solution containing 15.8% borates. Borax costs $.54/lb in a U.S. supermarket, boric acid costs about $4./lb in U.S. drug stores, but boric acid roach poison, about 99% boric acid is cheaper in discount stores. To make this solution, mix required quantities and heat until dissolved. Boric acid in particular, dissolves slowly. A stable solution (no crystals) can be stored overnight in a refrigerator and costs a small fraction of Tim-Bor and Ship-Bor.

Home-Brew Glycol Solution of Borates:
This solution consists of 50% glycol antifreeze, 28% borax and 22% boric acid. To make a stable solution, mix ingredients and heat until it boils gently. Boil off until thermometer shows 260o This removes most of the water of crystallization in the borax. This solution is stable at 40o (in the refrigerator) and has a borate content of 26%. Dilute the solution with an equal volume of glycol antifreeze to make fluid enough to use handily. With antifreeze at $6/gallon and borax and boric acid as listed above, this solution costs about $15/gal compared to Bora-Care at $70/gal.

This article appeared in the Fall issue of Classicboat and has been reprinted with permission of Herb Anthony, Editor and Commodore.