Why do these cars like taking money from your wallet

I have a small leak coming from the side of my radiator, looks like its coming from the top and i think its been there a while.
Has any body tried a rad seal product and found it to do the job ? i do have wynns rad seal but it says dont put it in the expansion bottle, so thats out. i dont want to put anything in the coolant just in case in seals the wrong thing.
thanks Peter