Heater core bad?

Stop leak products work best when added to clear water. Only way to get all the coolant out and clear water into a heater core is flush it. Flushing it could increase the leak.
Fact... Anything mixed with water in any containment will separate and settle to the lowest point. That's why majority of a stop leak product will end up accumulated in the lower rear of the block. Bottom line... try it if you want but don't bet on it.