The window sills in my screened porch are beginning to rot. The porch is four years old. Originally, the sills were covered with tile to protect them from the weather. The weight of the tiles and the mortor cracked and heaved the first winter. Last spring, I had the tile removed and the sills rebuilt. The sills were stained and coated with polyurethane. I coat with polyurethane each spring. The sills are beginning to split and rot . What can I do to prevent them from getting any worse?