If the leak it at the pump end, check for cast flashing and make sure it's clean & smooth.
If at the radiator, check that the outlet tube is also clean and round and not dented. Any low spot in the neck will not see pressure from the clamp.
Check that the inside of the hose is smooth and has no scars from previous fitting.
Fit the hose clean and without any sealant. Make sure the inner spring is not between the hose and the neck.
Fit the hose clamp just behind the neck's bead.
I have never used or needed sealant, nor gave I ever seen it suggested in any manual.