While doing an extensive old house restoration we discovered a great way to restore rotted window sashes. After cleaning out all the rot from the sash. totally saturate the area with 1/2 Gorilla glue and 1/2 acetone. After it has cured clean out the excess glue, fill with exterior wood fill, sand and paint! I repaired several old sashes this way five years ago and they are holding great! One was almost as light as balsa wood and there is absolutely no sign of deterioration. Hope this helps someone.