Yes, that window can be repaired. I have done about 70 so far this year in the Lake forest area. You will need to first take the window out and remove all rotten wood. Then cut a new piece, but not exactly the same as original. The originals don't allow for water to drain properly so you want to put a 10 degree bevel on the bottom of the jam. Also. You have black gasket material around the glass on the outside. You want to cut that out and caulk it in with high quality caulk. The manufacturers seal leaks and all the water pours into the lower sash. Mill your new piece and cut to fit. Bondo any large holes, sand to smooth, prime the entire window with all hardware removed. Paint and re-assemble.
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