Epoxy is made up of two components that harden when mixed together. With a consistency like dough, it can be moulded by hand but will hold its shape as it hardens. It bonds well to timber and holds up against weather. When dry, the filler can be sanded, primed and painted. And it also takes stain in a similar way to softwoods such as pine. Before staining, create fake grain lines by cutting into it with a utility knife. The timber-like illusion helps to disguise the patched-up work.
STEP 1 : Remove the rot
Probe with a narrow chisel or screwdriver to find damaged areas (rotten spots are noticeably softer). Rot often occurs under paint, so don't assume it is sound because the surface looks good. Gouge or scrape away large chunks with a chisel.
STEP 2 : Soak the timber
Apply timber preservative. Repeat until the timber won't absorb any more (usually three to four applications). Wipe away excess. TIP: Drill holes into the area to improve penetration. Filling holes is easier with a nozzled squeeze bottle.
STEP 3 : Apply the epoxy filler
Rebuild missing areas with epoxy filler. For deep holes, drive screws partly into the timber to anchor the epoxy. Smooth filler with a putty knife before it hardens, dipping the knife in epoxy solvent or lacquer thinner first to stop the filler sticking
Protect timber from rain for a week or so before applying epoxy. Rough up the timber with coarse sandpaper or a wire brush - epoxy sticks best to bare, rough timber. Paint exterior repairs within three days as UV rays degrade the epoxy. Soak severely damaged timber with liquid epoxy. Use a brush and squeeze bottle to push it deep into the area. Apply several times while the previous coat is still tacky.