Before filling any rotten areas, it's important to remove as much loose and rotten wood as possible. We recommend treating the affected area with Ronseal Wet Rot Wood Hardener first to stabilise and harden the damaged and immediately surrounding areas. Once treated and prepped, any rotten or damaged areas can be filled using Ronseal High Performance Wood Filler. Once this has set hard the repaired area should be sanded smooth ready for applying an exterior wood primer and paint.