Why Wood Decays
The fungi that destroy wood have simple needs: Air, water, moderate temperatures, and not too much sunlight, thank you. Whenever the moisture content of the wood in your home is greater than 19 percent (it feels damp to the touch), ever-present fungal spores begin to grow. As the wood dries out, the fungi go dormant, but they don’t die.
To prevent fungal growth, keep your siding and trim well painted, and keep trees and shrubs pruned to maintain good ventilation around the house.
To repair the problem, replace rotten boards with well-primed stock.
The heartwood in redwood and cedar are naturally resistant to rot. These species command a premium price both for their looks and durability. In many parts of the country, pressure-treated lumber is a more economical choice for fencing and decks. This lumber is impregnated with a chemical that wards off fungal decay and wood-destroying insects like termites, carpenter ants and powder post beetles.

Recognizing Wood Decay
The first visible sign of wood decay is usually a mysterious hole or deteriorating surface area in exposed wood. Investigate as soon as you notice those signs. All you need is a screwdriver. Poke all around the wood with it; if it goes through, probe further to see how deep and extensive the problem is.

Hardening Decayed Wood
If it’s impractical to replace decayed wood, you can consolidate the wood by injecting it with two-part (resin + hardener) liquid epoxy, sometimes called wood hardener. Drill small holes no more than 1 inch apart throughout the affected area. Angle the holes down on vertical surfaces. Dry the wood thoroughly with a heat gun. Mix the epoxy according to manufacturer’s instructions. Wet and rewet the wood with hardener until it is saturated. After the epoxy cures, apply an epoxy filler to hide the holes.

Fillers for Decayed Wood
To fill cavities created by decay, use epoxy-resin or polyester-resin fillers. Epoxy fillers are available at marine supply stores as well as some lumber yards. They bond better and shrink less than polyester fillers. Epoxy fillers cure quickly. You can tool them like wood, though this is far easier if you do it while the filler is still hardening.
Begin by removing decayed wood with a chisel. Mix the filler in a disposable container, and apply it with a stiff plastic squeegee or plastic putty knife. Do not exceed ½-inch thickness per application. Excessive heat from the curing process can ruin your work. As soon as the filler firms, shape and smooth the patch. When the filler is cured, sand it with progressively finer grit sandpaper.

Pros’ tip
For a better bond, pre-wet porous surfaces with a liquid epoxy before applying a epoxy filler.

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