Rotten exterior siding means water got behind the paint film. Cut back to solid wood, fix the water path, then patch—do not paint over soft fiber.
Find the water source
Check gutters, kickout flashing, window sills, and grade splashback. Repairs that ignore the leak fail within a season.
Remove soft wood
Probe with a screwdriver. Dig or cut until you reach firm fiber. For large areas, replace full courses of siding rather than stacking filler.
Consolidate or replace
Small pockets can take a wood hardener and epoxy fill after drying. Structural or wide rot needs new boards flashed and caulked correctly.
Prime and paint
Prime bare wood the same day you expose it when weather allows. Use exterior-grade products and maintain caulk joints at butt ends and trim.
For more repairs like this, start in the Rot & Outdoor Wood hub. If you are working through a related problem next, see Wood Decking Repair: How to Fix Cracked and Rotted Deck Boards or Wood Deck Surface Repair (Professional Tips for Outdoor Maintenance).
