Replacing the heater core will be much less of a headache in the long run then putting that crap in your cooling system. I would not recommend putting any stop-leak products in your cooling system, ever.
I have been using Bars Leaks dry "stop leak" pellets on every thing I own, even when I replace the entire cooling system. Been using it on everything for about 40 years now with the old school green coolant. I never have any problems
except when I buy a used a rig that has been abused, LOL. Those I fix and add bar leaks dry pellets to. I usually keep my rigs for 300,000 to 400,000 miles and about 15 years.
Now for the OP. It may work, and it may not. Depends on the size of the hole. How long it will work on an existing hole depends on how bad it is.
Detroit has been using dry pellets like Bar leaks pellets on all new cars (in the brand new radiator tank when brand new, since at least the 1930s according to Prestone. That is how Detroit avoids warranty cooling system leak problems, LOL.
The guy that first taught me car mechanics in 1974, was a race car mechanic and dealer mechanic for 40 years, going back to the 1930s. He taught me the cracked pepper trick and the history of it.
Race car drivers use to use cracked black pepper at the race tracks it they got a last minute leak. They are both an organic fiber that seeks out leak-holes. They collect at the hole, get stuck in the hole and accumulate till they seal the hole. And ingredient in all coolants combines at the hole with the fibers to seal the holes.
