Wood Finishers, Wood Sealers, Protecting Wood Finishes

What’s the difference between a wood protectant and a wood preservative? A big one, according to the Environmental Protection Agency (EPA). In order to be described as a preservative, a wood-finishing product must be registered with the EPA to certify that it is effective and environmentally acceptable.

“Depending on what type of wood preservative the product is, there are different classifications that the EPA requires, based on what claims you can make,” explains Steve Revnew, director of marketing for Sherwin-Williams.

Various preservatives protect against one or more of the following: mold, mildew, ultraviolet (UV) light, insects such as termites, and fungi that cause rot or decay.

Even though they are designed to protect wood surfaces, most stains are not registered as wood preservatives. They protect the wood surface primarily by improving water repellency. Other qualities, such as UV and mildew resistance, are typically formulated to protect the stain itself, rather than the wood.

“Almost all oil-based stains are made with vegetable oil, which is a food source for mildew,” says Jake Clark, owner of Armstrong-Clark Co. “Raw linseed oil doesn’t have a mildewcide in it (like boiled linseed oil does) and the mildew just loves it. If you make a coating out of a vegetable oil, you have to put in a mildewcide to keep it from being attacked by mildew.”

Water-based stains can also be formulated with a mildewcide and a UV blocker to protect the finish. “More and more people are going more towards the water-based products,” Revnew says. “They have longer-lasting color and gloss retention because they’re typically made up of an acrylic-type resin. Water cleanup is also very desirable.” He says that oil-based stains tend to chalk and fade at a faster rate, but they provide better water repellency.

Top: Before Bottom: After

Several choices are available for projects that require a wood preservative. One is to use a stand-alone wood preservative before applying a stain. “Normally, preservatives are oil‑based systems and can be recoated,” says Peter Hope, technical specialist with Samuel Cabot Inc. Water-based stains can be successfully applied over an oil-based preservative. “It is important to check with the coating manufacturers for specific instructions. Cabot has application instructions on their labels,” Hope says.

Some preservatives can be applied on-site, either by spraying or brushing. One example is Green Products Co.’s Copper-Green Wood Preservative, which contains copper naphthenate. Another is Shell-Guard, a glycol/borate formulation made by Perma-Chink Systems Inc.

A second option is using wood that has been pressure-treated with a preservative. Some stains are designed to counteract the characteristic greenish color of pressure-treated wood. Superdeck 2000 Series Transparent Stain for Pressure Treated Wood is an example. “This product is designed to work with the color of treated woods to result in an attractive stained look while retaining the grain clarity of the natural wood,” says Sarah Key, marketing coordinator for Duckback Products Inc., which makes the Superdeck line. The line includes a wood cleaner that can be used before applying stain to pressure-treated wood that has weathered.

A third option is to use a stain that includes a preservative. Sherwin-Williams’ Exterior Alkyd Wood Preservative, an oil and solvent formula that is available in semitransparent and solid color versions, is such a product. “The wood preservatives protect against decay of the wood as a result of water infiltration,” Revnew says. “Typically, wood preservatives are used more on horizontal surfaces, because you’re going to have pooling water sitting on it.”

Woodguard, another line of broad-spectrum preservative-containing stains, includes transparent and semitransparent versions. Developed by ISK Biocides Inc. for use on log homes and log siding, the products are available in six standard tints that can be further customized by the dealer.

In damp regions such as the Pacific Northwest, both new and stained wood should be treated to kill any existing mildew before applying a stain, even if the stain contains a preservative-type mildewcide, Clark cautions. “Mildew is unsightly,” he says. “The preservative may kill it, but you still could see it.”

Before applying a stain in that kind of climate, new and previously stained wood are both typically treated with a mildew wash consisting of chlorinated bleach, water, and possibly an additive that increases the bleach’s effectiveness.

Stains enhance the appearance of wood and protect its surface. But unless a stain has earned federal registration, it can’t claim to preserve the wood from rot, decay, wood-devouring insects, mold or mildew. Protect or preserve — thanks to the EPA, it’s more than a matter of semantics.