A Rotted Window Sill !link!: Repairing

Run a bead of exterior silicone or acrylic latex caulk along the seam where the sill meets the window frame and the siding. This keeps water from seeping back into the repair.

Remove loose paint and caulk around the sill. Use a chisel or utility knife to probe the wood. Soft, spongy areas are rotted. Mark where the wood is solid. repairing a rotted window sill