Back in the seventies I was employed as a radiator repairer, we used to strip the tanks off and fit new cores in them, in those days they were made from brass and copper.... the tubes which the water runs through are very narrow and using products like radweld and sealit etc stop small leaks simply by blocking them up, as do flour, oatmeal and eggsetc.... yes, we had that too stank to high heaven.....
these products are really best for a temporary repair (although having seen the results during rebuilds I would never even contemplate using them in my cars)
The best and only sure way is to replace the radiator in these days.
We used to cut the leaking tube out by using a hacksaw blade and cutting it at the top and bottom plates and then soldering the holes up, but of course this is not posible on the modern plastic types used today, (plastic killed my job)
HTH
Pete