Water is your friend! Fill your heater core up such that only the portion you're working on is dry. Depending on how close the water is, it will take longer to reach solder temperatures or you may need a hotter torch (MAPP will definitely do it).
A waterproof epoxy will work better than GOOP for leaks you can't solder. Make sure the area is clean and dry, and use a little suction on the barbs to pull the epoxy into the wound.
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