The wood was literally crumbling as I tried to sand it, and the middle core of the wood was crumbling too....I had to make a decision on what to do about this piece of wood. I decided that it was a lot less stress to just recreate the piece with some basswood strips that I have.
First I used my mitre box to mitre the corners of the wood.Next step was the glue the pieces together. I used my jig to make sure that the pieces stayed square. This particular window frame was for a double window, so I had to also glue in a middle piece of wood, so I used the window sashes in the frame to help me know exactly where to glue the middle strip. Here's the final result. Looks much nicer than what Greenleaf provided.