Wooden windows are not invincible. Although they can last for decades on a few coats of paint, they will eventually crumble under the elements if they are not maintained properly. The two main areas of an exterior wooden window that are prone to rot are the window sashes and the window sill. These are vital for window pane support and structural integrity. If your window has rot within its wooden components, you will need to repair the window before it completely fails.
Carry out any repairs on a wooden window while the window is removed from the frame. All wooden windows have trim that is attached around the inside of the window sash. The trim holds the window inside the window frame and allows the window to be raised and lowered easily. The trim must be carefully removed in order to pull the window out. You will not be removing the entire window frame from the wall opening, only the window sash that holds the window panes.
Once the window sash has been removed from the frame, lay it onto a table or work surface. Place a towel on the work surface and lay the window on top of the towel to help prevent any further damage to the window.
Once the window has been removed from the frame, you can begin repairs on the window sill, as well. The sill is located on the bottom of the window frame and extends out toward the exterior of the wall. Most of the time, the window sill will receive the most rot damage because of its location relative to element impact; the sill receives direct impact from rain and sun. Remove the sill carefully using a flat-bar so you can trace the old sill onto a new piece of wood. Use a jigsaw to cut out the new sill and attach it into the window frame opening with finish nails.
When replacing the wood on the window sash, you might need to have a piece specially milled. Be careful when removing any wood rot from the sash; you could break the window panes in the process. If the rot is minimal--2 inches square--you can patch without removing the wood component. Use a screwdriver to remove any damage by scraping and digging away the rot. Use exterior wood filler to fill the void. After completing the wood repairs, sand the wood filler, prime the wood and repaint.