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Rot happens, even to the best of houses. All it takes is wood, water, and warmth, and before you know it solid lumber turns to mush. Exterior trim is the most vulnerable to attack by rot fungi, and it doesn't have to be very old; the trim shown here was installed only 10 years ago.

Fortunately, rotted trim is generally easy to repair. (Rot-infested framing or mudsills pose a much bigger problem.) But before you can fix it, you have to find it. Check out the horizontal areas that don't drain well and places where the paint is cracked, peeling, or blistering or the wood is darker. If your screwdriver pushes deeply into a suspect board, it's time to root out the rot. Pay particular attention to joints, which dry slowly, and to all wood that's close to dirt, concrete, or masonry.

For a relatively confined area, a two-part epoxy resin is a smart option that yields a seamless repair for pros of all levels. Here, John Stahl of Advanced Repair Technology, who restored the old windows for This Old House TV projects in Milton and Salem, Massachusetts, takes us through a typical repair of a rotted window mullion.

Step 1

Remove Rot-Softened Wood

Photo by Brian Wilder

After clawing out the loose stuff with a hammer, Stahl removes all the rot-softened wood with a die grinder and core-box router bit. For an epoxy repair to be effective, the freshly exposed wood has to be sound and dry—less than 18 percent moisture content. Stahl checks it with a moisture meter before proceeding.

Step 2

Inject Borate Into Holes

Photo by Brian Wilder

The undisturbed area at the bottom right of the mullion is an old epoxy repair, around which the wood continued to rot. To ensure that won't happen again, Stahl injects a borate wood preservative into holes drilled halfway into the wood. Sealed over with epoxy, the borate penetrates the wood, minimizing the chance of future decay.

Step 3

on the exposed wood

Photo by Brian Wilder

A two-part epoxy primer brushed on the exposed wood ensures that the final repair will bond to the surface. After waiting about 15 minutes for the thin liquid to penetrate, Stahl wipes off the excess with a paper towel. The surface is now ready for a coat of the two-part epoxy filler.

Step 4

Blend the Resin and Hardener

Photo by Brian Wilder

Stahl pumps the two components of the epoxy filler—resin and hardener—onto a plastic board, then blends them thoroughly with a plastic putty knife. Epoxy doesn't stick to hard plastic surfaces, so the board and putty knife can be cleaned and reused.

Step 5

Sculpt the Epoxy

Photo by Brian Wilder

Using the same plastic putty knife, Stahl sculpts the viscous epoxy into shape. The mix remains workable for about 30 to 45 minutes (longer in cool weather and shorter when it's hot).

Step 6

Paint the Epoxy

Photo by Brian Wilder

Epoxy breaks down in sunlight, so it needs to be painted. The next day, after the repair hardens, Stahl sands it smooth, first with 80-grit paper, then 100-grit, then 220-grit. An acrylic primer is next, followed by two coats of 100-percent acrylic paint.