It's hard to make epoxy repairs in there.
You need to get everything very clean and dry, and not damage the other (good) tubes, and access is tight. The leak may even be on a tube in an inner core, which is even harder to reach.
I have had better luck crimping off the leaky tube with needle-nose pliers. You can make multiple crimps on each side of the leak.
If it still leaks, you can cut the tube near the leak, and fold over both ends to make a very tough crimp.
Whether you use the crimp method or the epoxy, you will need to (carefully) sacrifice some of the fins in the area around the leak to gain access.
Keep in mind that one or 2 tubes and some fins is insignificant in the context of an entire radiator.
I once ran 40K additional miles on a radiator after a single 'crimp repair' with no problems. Not a bad 'life extension' for a free repair...
IMHO, those internal stop-leak products are a bad idea. A radiator that is already getting towards the end of its service life does not need additional gunk accumulating in it.
1986 300E 5-Speed 240k mi.