I suggest one of the polysulfide windshield/window sealants like 3M Aerospace Sealant AC-251. It's made for exactly what you describe. Mask off 1/8" on each side of the joint, mix it up and brush it over the gap with a 1/2 length acid brush. Pull off the tape, let it sit overnight and voila! Won't have to worry with it again. Wear gloves cause it takes ~2 months to get it off your hands.