Wood rot is induced by a plantlike organism similar to the fungus which causes food spoilage and mildew. Like all other plants, this organism can thrive only if moisture is present. That is why it is particularly important that wood which is in contact with the ground, or which is endlessly exposed to dampness, be correctly protected. For years the lone effective wood preservative available was creosote.
This bears numerous disadvantages. It leaves an objectionable, lingering odor, and it's got a dark, horrible color which is virtually impossible to paint over. Nevertheless, in later years new kinds of wood preservatives have surfaced which do away with most of the disadvantages encountered in creosote.
While these preservatives are sold under different brand names, most comprise of either pentachlorophenol or zinc naphthenate. They're sold as clear solutions and are often mixed with water repellant to give additional protection to the wood. They not just protect against rot and insect attack, they also bring down warping, checking and swelling. Some also serve as a primer for the wood so that they literally improve its ability to hold paint.
Pentachlorophenol (known as "penta" for short) is generally sold in the form of an oil-base solution which can be put on by brush or spray, or by soaking and dipping. Because the preservative need to penetrate to give utmost protection, soaking or dipping is by far the most effective means of application. Where practical, soak the bottom ends of fence posts and similar pieces by letting them stand upright in a can full of the liquid, or try placing long pieces into a shallow trough made of sheet metal or wood lined with plastic sheeting.
If dipping or soaking is impractical, the preservative must be brushed or sprayed directly onto the bare dry wood. Flood the liquid on, making sure to work it into crevices and into exposed end grain. If the piece is tiny, place a large tub or other container under the work to gather excess liquid which drips off. If you are going to spray the liquid, use a coarse, low-pressure spray. Be sure wear a respirator mask, and spray in a well-ventilated area only.
Wood preservatives which contain zinc naphthenate in a colorless solution are employed in the same manner as those which contain penta. Some of these are exceptionally clear and are made to act as a wood sealer too. They can be used under varnish if a clear natural wood finish will be applied.
Besides applying these wood preservatives to new lumber, the homeowner must also apply them to existing wood structures at sensitive places. For instance, preservative should be brushed generously onto the bottom and top edges of garage doors and entrance doors. Apply also in the joints of wooden porches and steps. Wooden gutters, shutters, storm sash, window frames and fence rails. All these will benefit from having wood preservative brushed onto exposed parts and joints when paint peels or cracks away.
To reach into tiny crevices and tight corners, like under the door sill, an ordinary pump-type garden sprayer or pressure-type oil can does the job. Keep in mind that wherever two pieces overlap or meet, a pocket is created where moisture is probably to collect and remain. These joints are especially susceptible to attack by fungus, so give them an extra dose at periodic intervals.
When applied to wood which has started to rot, the preservative won't restore the wood that is already decayed. Still, it will effectively stop the rot from fanning out if it is correctly applied. For best results, take out the section of decayed wood completely and replace with a new piece which has been exhaustively coated with preservative on all sides.
Apart from its use on the house itself, wood preservative may be used to protect ladders, picnic tables, garden furniture and similar items. Soak the bottom ends of the legs, and try to apply preservative into the joints where cross-members are secured together or where wood crosses wood. In these joints moisture gets trapped so that the wood continues to be damp for long periods, making conditions ideal for rot to develop. When putting on penta by spray, brush or dipping, homeowners should avoid contact with the skin because the chemicals will irritate most people.
Log Home Living February 1990
Popular Mechanics Magazine