To fill long thin spaces, cut some pieces of wood like popsicle sticks. Plane and sand them thin to a knife edge on one side. Then cut them to length and push them in with the tinest amount of glue. (You don't want to create another glue visual problem) Let them harden proud and plane and sand them off. They wil be invisable. I do it before any epoxy. Wet the area to make sure there's no glue traces.