Wood Rot Prevention Eight Years Later

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Wood is strong, stiff per unit weight, easy to work with, attractive, and economical. And it rots. A few special types of wood, such as teak, are highly resistant to rot, but these woods are generally heavy and almost always expensive. Pressure treated lumber is an option, but it’s ugly, warps, does not take coatings well, and is not always very rot resistant. Western red cedar, on the other hand, has long been used in canoes, and cedar fence posts have a deserved reputation for durability.

We started our rot resistance testing eight years ago in 2015, and we finally cleaned out our accelerated rot test box in July 2023. Results ranged from abysmal (untreated samples of fir have nearly disappeared) to astounding (our teak samples are still just like new). Let’s see what we learned.

WHAT WE TESTED

We tested fir, pressure treated pine, western red cedar, and teak. We tested samples of these woods that were treated with a DIY borate solution made of Borax and water (see “DIY Wood Preserver & Treating Teak” PS January 2021), a DIY solution made with Borax and glycol, and we tested samples—untreated and treated—that were protected with varnish or epoxy.

The DIY borate solution was about 25 percent by weight, or about one cup of Borax dissolved in a pint of hot water. The borate concentration can be increased by adding an additional 5 percent Borax. System Three End Rot and similar powdered fungi-fighting products are commercial versions.

Our syrup-like DIY borate-glycol solution was made by heating one gallon of ethylene glycol with one pound of Borax, dissolving all solids. Making our own solution instead of buying it saved about $100 per gallon, but because of the small but serious risks involved in creating our homebrew, we don’t consider it an ordinary DIY project. If you have an affinity for basic chemistry projects and are comfortable working with a respirator, the DIY formula is online at Practical Sailor (see practical-sailor.com/boat-maintenance/products-and-tips-to-stop-wood-rot). If you don’t mind spending your freedom chips on wood protection, Boracol and Bora-care are similar products and available online.

The advice to seal and protect wood with epoxy is familiar to any sailor who has spent some time at a local boatyard, so we were particularly interested in the effectiveness of epoxy or varnish in warding off rot. The old salts assure us that epoxy resin, sometimes followed by a coat of varnish, is a potent antidote to rot. Our testing does not bear this out.

Varnish has long been used to combat weathering. A chief advantage of varnish over paint is that the clear coat lets you see when rot is developing. Traditional varnish is also less likely to crack as wood expands with changes in humidity and temperature. Our clear coat of choice for this test was Epifanes Clear varnish. Formulated with tung oil, phenolic and alkyd resins, and UV filters, this traditional varnish has consistently done well in previous varnish tests (see “Exterior Wood Finish Update at 2 Years,” PS July 2021).

HOW WE TESTED

To simulate a worst-case scenario, we placed the samples in a sealed plastic storage bin with fresh water saturated compost and left them for eight years. We wanted to simulate the most horrific bilge imaginable. Our compost heap was truly a horror show. It hosted much more bacteria and fungus than a typical bilge, and our containers did not benefit from the presence of saltwater, which can inhibit rot. Based on what we’ve observed over the years and learned from the results of independent tests, the eight years in our wood torture chamber were the equivalent of about 15 to 25 years submerged or exposed to a damp environment.

At the start of our test, the sealed bin was originally full of firm wood compost, but by the end test, the compost was practically liquefied. Some of our wood samples had virtually dissolved.

The Achilles heel in most samples were screw holes. We placed six screws in each sample and this effectively pierced the coating. Why do this? A screw hole or two is nearly inevitable when working with wood on a boat. Unless you pre-drill and seal every hole, wood grain will be exposed.

Because we happen to have a totem pole carved from jack pine on our property in Virginia, we also treated the base of the totem pole with the borate-glycol and painted it with copper bottom paint. Coating the base with bottom paint—we hoped—would seal the borate treatment and repel insects.

OBSERVATIONS

Monitoring this project has been a labor of love and a not-so-subtle reminder of the inevitable effects of time. As the years passed, it seemed our search for an elixir to cure wood decay would be no more fruitful than Juan Ponce de Leon’s search for the Fountain of Youth.

Coatings. The samples of untreated fir that were coated with epoxy or varnish disintegrated completely, leaving only the coating shell behind and a few bits of mulch. The only penetrations were a few small screws, so unless you can guarantee a flawless coating that won’t chip or crack, you can’t rely on coatings alone for rot protection in wet environments.

Samples that were treated with borate-glycol and then coated with epoxy or varnish did much better over the long haul. Once the anti-rot treatments have soaked in, the coating’s main function is not to keep the wood dry, but to keep the treatment from washing out.

DIY borate and borate-glycol treatments. Our DIY formulas slowed the progress of rot, but the treatments without coatings did no better than lumber that has been pressure treated for Use Category 2, damp interior environments (UC2). As we describe below, the very common UC2 pressure-treated lumber was no match for our torture chamber.

Treated samples that were also coated with epoxy or varnish held up a little longer than samples that were protected by the borate treatment alone. The coatings appear to keep the preservative chemicals from leaching out. The level of protection was still unsatisfactory, however, so you are much better of choosing a rot resistant wood, such as red cedar, teak, or mahogany.

Western Red Cedar. Western red cedar contains a rot-preventing oil that is not easily washed out. This is the “tree of life” of the Pacific Northwest providing everything from clothing to medicine for the many tribes of that region, even today. And of course, it was the wood of choice among these cultures for building their famous rugged dugout canoes.

In our quest for a light, relatively inexpensive and completely rot-proof wood, we applied our DIY borate-glycol treatment plus varnish to a large cockpit grating made of rot-resistant red cedar. Although teak is normally used for cockpit grating, we chose western red cedar because teak would have been too heavy and expensive, and because we liked the way the cedar looked.

Additional samples were treated with our DIY borate-glycol, or the DIY Borax-water solution. Varnish adhered very well, but the epoxy peeled away. In our testing, just two coats of varnish proved more durable than epoxy. For exterior use, you’ll want additional coats of varnish for UV resistance.

So far, the triple protection of rot-proof wood, Borax-water treatment, and varnish has performed perfectly. The grating has been revarnished just once, and it is still free of rot. The borate-glycol treatment does not appear to inhibit gluing or painting. We have used some leftover treated strips for a few boat projects and have experienced no problems coating or bonding (see “Products and Tips to Stop Wood Rot,” PS January 2021).

Only the untreated, uncoated sample of western red cedar showed any deterioration, which was limited to a small area of end grain. The samples did not warp or soften.

Jack Pine Totem Pole. The buried base on our jack pine totem pole is still going strong, with no noticeable rot. We renew the Borax treatment every 5 years by pouring Borax into deep holes drilled at an angle, down into the base. This method is commonly used by power companies to protect older power poles that were creosote treated. While our test was ongoing, some similar, slightly younger totem poles carved from local pine and installed at a nearby botanical garden park fell over in a storm; their bases had rotted out.

Pressure Treated Wood. Pressure treated lumber is classified by use categories (UC), ranging from UC1 to UC5c. UC2 is suitable for interior applications that might get damp, such as sill plates and furring strips for installing paneling or drywall in a basement. UC4 is required for wood that makes contact with the ground, such as deck posts. UC5c is suitable for continuous immersion, including dock boards and pilings.

We tested furring strips (UC2c) because they are the most common kind at the big box home stores. Furring strips are typically only available in UC2, so if you need smaller dimensioned lumber, we suggest ripping it down from UC4 2x6s. Our UC2 furring strips failed miserably, demonstrating that lower category pressure treated lumber is not very useful on boats. Even before they rotted, they warped and became soft.

Teak. Renowned for its resistance to rot, teak did not disappoint our testers. The drawback is its high cost as well as environmental concerns over the impacts of harvesting the tropical hardwood—even when the wood originates from so-called “sustainable” forests. Teak and holly, the traditional combination for a teak sole, makes a beautiful combination, but holly is not a marine wood. In our testing, the holly began to rot in the first year, turned to mush within three years, and it was gone by the eighth year.

CONCLUSION

Treating wood with a Borax solution and then sealing it with varnish will help fight off rot, but there is no substitute for inherently rot-resistant wood. Teak is the gold standard. Western red cedar performed very well, costs much less, is lightweight, and widely available. It is not completely impervious to rot, but a good coating of varnish seems to hold the rot-inhibiting oils in place, making it quite durable, even in the worst imaginable conditions.

Unfortunately, red cedar, like teak, is a victim of its own success. Despite protections, valuable cedar forests—some containing trees more than 1,000 years old—are still being slated for harvest in the U.S. and Canada.

Pressure treated wood from the big box store did not impress us. Yes, it was low-grade UC2, but even the higher-grade timbers will warp.

We found no evidence that epoxy alone offers exceptional rot protection in a saturated environment. We’re tempted to regard its mythical protective properties as just that—part of a myth that persists in the face of all evidence to the contrary.

Why doesn’t it work? First and foremost, the epoxy seal will almost certainly be breached by a fastener or a crack as the wood expands and contracts. Water contaminated with bacteria and fungus will enter, and unless the wood was rot-resistant to start with, the wood can turn into mulch in just a few years.

This is not to say that epoxy has no value in dealing with sealing or protecting wood structures on boats. For example, epoxy is still the go-to method for sealing plywood end-grain (see “Medium Density Overlay in Boats”) In the future we will be carrying out more testing to explore the value of epoxy-sealed wood, and the various types of epoxies used for sealing wood.

PS Test Replicates Decades of Exposure

We placed a variety of wood samples, treated and untreated, in a sealed plastic storage bin with fresh water saturated compost and left them for eight years. The aim was to create a perfect environment for rot, effectively accelerating the exposure period. By the end of the test, it was obvious which woods best resisted rot, and which preservative treatments showed promise. Based on what we’ve observed over the years and the results of previous tests, our test mimicked the equivalent of about 15 to 25 years submerged or exposed to a damp environment. While the most rot-resistant woods came out intact, some of the test samples, even treated samples, had turned to mulch.

Wood Rot Prevention Eight Years Later

1. Replenished with water throughout the test period, our swampy test bin accelerated the decaying process. At the end of the tests, some of the samples had effectively turned to mulch. It’s no mystery why balsa decks get soft once water is allowed to enter.

Wood Rot Prevention Eight Years Later

2. Three western red cedar boards. The left was treated with borate, the center untreated, the right with glycol. All three were completely sound.

Wood Rot Prevention Eight Years Later

3. Fir treated with Borate (left) or glycol (right) and then sealed with epoxy was weakened about 50 percent, but was still generally sound.

Wood Rot Prevention Eight Years Later

4. Pressure treated fir and fir are not in the same category. From left to right: fir treated with glycol only; pressure treated fir; pressure treated fir (from a different batch); deck tiles coated with epoxy; fir treated with glycol and varnished; untreated fir.

Rot Prevention Rogues’ Gallery

Our initial research into various rot prevention treatment was inspired by a time spent in the Pacific Northwest, where the National Park Service used a borate solution to treat totem poles. After a few tests on various types of wood, our research into the effectiveness of borate on boats began eight years ago. It didn’t take long to settle on a couple of DIY formulas—one mixing Borax and glycol and a simpler version made Borax and water. Both are much cheaper than commercial products, (see “DIY Wood Preserver & Treating Teak” PS January 2021).

Wood Rot Prevention Eight Years Later

1. Untreated totems of the same age near our test location came down easily in a winter storm. The black area is rot, not fire damage.

Wood Rot Prevention Eight Years Later

2. A power pole not far from our test location has a badge indicating the year of its last borate treatment.

Wood Rot Prevention Eight Years Later

3. Untreated western red cedar shows some weathering at the end grain, but the wood is still fully intact.

Wood Rot Prevention Eight Years Later

4. Untreated teak demonstrates its exceptional resistance to rot.

Wood Rot Prevention Eight Years Later

5. A teak and holly sole looks beautiful, until you immerse it in fungi soup for eight years, when the holly turns to mulch.

Darrell Nicholson
Darrell Nicholson is Director of Belvoir Media Group's marine division and the editor of Practical Sailor. A lifelong thalassophile, he grew up sailing everything from El Toro dinghies to classic Morgans on Miami's Biscayne Bay. In the early 90s, he left a newspaper job to sail an old gaff-rigged ketch across the Pacific and has been writing about boats and the sea ever since. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida.