A reader wants to know how to preserve this chain-sawed wood carving. (Reader photo)

Q: When the white oak holding up half the hammock at my vacation home died, I did not want to move the hammock. I hired a talented local chain-saw artist, Andrew Mallon of Falls Church, Va., to carve the stump into a beautiful osprey and heron. It has been stained and varnished and looks wonderful. I know eventually the stump will decay, and I will have to cut off the carving and put it on some sort of concrete base. I want to stop the rot for as long as possible. What can I do to delay the inevitable?

A: To keep your carving in good shape for a long time, you have two issues to deal with: rain and the sun.

Wood that’s out in the weather can last without rotting if it dries between storms. But persistently damp wood is prime habitat for fungi that cause it to rot. That’s why fence posts that haven’t been treated with preservatives rot out at the base, where they are surrounded by damp soil, while the aboveground part of the post still looks fine even though it gets rained on.

To keep the stump under your carving from rotting, try drilling holes near the base and inserting rods made of borate, a mineral that stops rot and also controls termites and carpenter ants. Borate was registered with the federal government as a pesticide in 1948, but it was discounted as an effective wood preservative for many decades because it is water-soluble. It eventually leaches out of wood that’s sitting in water, which made it seem like a pointless way to try to protect outdoor wood. Eventually, people realized water-solubility can be a benefit when wood is partly wet and partly dry. Borate moves with the moisture to protect the wood where it is most likely to rot. The mineral not only prevents decay, it even keeps rot that has already begun from getting worse.

Borate rods have been used successfully to preserve everything from totem and telephone poles to log homes. Bor-8 Rods, formerly called Impel Rods, are pure borate and look like pieces of clear glass dowels, while Cobra Rods also contain copper, which the manufacturer says boosts their ability to fight rot. They also look like glass but are dark green. Bor-8 Rods come in diameters from one-quarter to three-quarters of an inch and lengths from half an inch to four inches, while Cobra Rods are about half an inch wide and two, three or four inches long.

Western Log Home Supply (970-315-2660; westernloghomesupply.com) sells both types. A box of 50 rods half an inch wide by two inches long costs $199.61 for the all-borate version and $89.99 for the borate-plus-copper type.

Companies that sell the rods also supply charts listing appropriate sizes and spacing for different diameters of wood. Drill about at the ground surface, angling downward toward the center of the stump. Then hammer one or more rods into each hole, as specified in the chart, and cap. After a few years, you can pop off a cap or two and inspect. If the rods have dissolved, tap in new ones so the protection continues.

The safety sheets for these rods warn against eating them, rubbing your eyes when handling them or throwing them into waterways. Otherwise, there is little known risk to you or the environment.

But protecting against rot isn’t your only issue. To keep your carving looking good, you also need to protect it from sun damage. Ultraviolet rays break down lignin, the component in wood that holds the fibers together. That’s why unprotected wood becomes gray and rough — the surface fibers are coming loose.

Mallon, the Falls Church sculptor whom you hired (703-919-4835; amsculptures.com), said he always applies two coats of spar urethane, specifically Minwax Helmsman Satin Oil-based Spar Urethane Varnish ($16.98 a gallon at Lowe’s). He recommends applying a new coat of finish every year or so, preceded by a thorough cleaning. He uses a scrub brush and rinses with a hose, never a power washer, and waits for the carving to dry before he applies the new finish. He uses a brush rather than a sprayer because brushing results in a thicker coat.

There’s good science to back up this protocol, said Tom Lyman, director of product development for Sherwin-Williams, and Julie Vojtko, director of marketing for several of the company’s brands, including Minwax. There are basically two ways clear or transparent finishes can thwart ultraviolet rays. Tinted finishes often contain UV blockers, which are finely ground particles that physically interrupt the wavelengths of light that do the damage while still letting through most wavelengths in the visible light spectrum. These particles offer permanent protection to the wood as long as the finish doesn’t wear off, but they do add some color. Clear finishes, including the finish on your carving, depend on different chemistry. They have UV absorbers, which protect the finish and keep it intact, thus indirectly protecting the wood underneath. However, the absorbers get used up as they react with light. So you need multiple coats at first and new ones periodically. The fresher the finish, the more protection the wood gets.

Mallon uses a solvent-based urethane, and Vojtko recommends sticking with that type, although she said you could also switch to a water-based version, such as Minwax Water Based Spar Urethane ($18.48 a gallon at Lowe’s). Both formulas provide equal protection with UV absorbers, she said.