You are in trouble---Generally if the sash is rotting the door is done--junk.
Post a picture if you can---I repair a lot of rotting window sills but the cost in time and skills and tooling to rebuild a sash has always made replacement the better option.
Post a picture if you can---I repair a lot of rotting window sills but the cost in time and skills and tooling to rebuild a sash has always made replacement the better option.