Bill,

Polyurethane is the best. (Vulkem 911 is a brand I have used, but there are several available) It is not very easy to work with, but if you'll mask off both sides of the joint with tape and be careful, you'll do a nice job. Overfill the joint slightly and then, using a putty knife held at a very low angle, smooth the surface flush with both sides. (while the tape is still there).

Takes a while to cure and not easy to clean up, but, taped carefully, you can do a very professional job.