Once you've got the JB Weld to the desired level, wait until it begins to "sag" then give everything a finish wipe and clean and set the knob aside for 16h or so. There will be a little settling as the stuff cures, and you can clean up any overlooked runs or splotches as long as it's still liquid.

After full curing is attained, place knob back on shaft and secure with set screw. If done properly you'll break the radio before you break this repair.