To fix dry rot in wood, patch non-weight-bearing wood surfaces with epoxy or polyester resin wood filler. This approach requires removing loose and crumbling wood fibers and applying wood hardener and wood filler to rebuild the rotted area.
Depending on the depth and extent of the damage, use a wire brush or another sharp tool to remove rotted wood. For better results, blow out dust and small wood particles with a shop vacuum. Brush a wood hardener product liberally onto the remaining affected area, and let it dry for two hours or as recommended by the manufacturer instructions. Next, use epoxy or polyester wood filler to fill in the rotted area. Bondo is one example of a two-part polyester wood filler that dries very quickly.
If necessary, create a form with scrap wood to hold the filler. Use plastic or a lubricant to line the inside of the form for an easy release after the filler dries. If no form is used, when the filler starts to harden (about 10 minutes), cut off any excess with a utility knife or chisel, or shape it with a rasp or sandpaper. Most medium-to-large repairs require two applications of wood filler. To complete the project, sand, prime and paint the repaired area.