Filler(interior) for timber window

My 70+ yo house has two west facing timber windows. Each comprises two smallish sash windows separated by a large fixed pane. There has been some minor rot along the bottom rail of the large pane where it meets the glass (no doubt from condensation). A previous "fix" seems to have been the use of wood filler. This has now well broken down down and some further very minor rot has occurred. I have cleaned out the old filler and the rot and applied wood hardener. The "groove" is about 5mm wide x 5mm deep. There is still plenty of the rail left to provide a level.

I am seeking advice on what filler use. The windows have been stained but will now be painted.



Many thanks