Used the powdered aluminum radiator leak-stopper around 2005 on my Honda Steed 400 VLX bike. It stopped the leak... most of the time. By most of the time, I mean it won't leak for several months and then one night, I can see a few drops of coolant underneath. Just topped it up again the next morning and it won't leak again for several months.

I went with this approach because the radiator is already unrepairable (as advised by half a dozen radiator repair shops I showed it to), and a new one would still need to be ordered abroad.

5 years later, the bike still runs great. I still see a few drops of coolant every few months but that's it.

Now, I'm not saying it will work for everyone. And I would never think of using it in my car. But it obviously worked on my bike.