I have used J-B weld on my car radiators in the past. It works. But if that's too runny, there's the JB weld steel stick. Just take a piece of that and mush it up in your fingers and stick it on the radiator like you're sticking chewing gum on it.
I have heard of used yes black pepper and also cranking an egg into the radiator. I have used Bars Leak and it worked temporary just to get by. No idea if permanent. I've used JB weld Water weld and it at least slowed down till I got a new radiator.
I have seen the black pepper done but I've only seen the egg in the radiator on MacGyver when I was a kid so I wasn't sure if that worked or not. The Bars Leak Copper works really good. I've had it in a Ferguson TO-20 tractor of mine that the wet sleeve Z120 block is split between every cylinder yet it doesn't leak a drop into the oil pan!
