What they said. I like to cut corners sometimes but I won't use that crap in anything unless it's to buy some time before driving it into a the scrap heap. Even then I wouldn't expect it to fix an external leak, especially in a heater core. If it did magically fix the leak it'd probably also plug up the rad and heater core as well.
I'd just bypass the heatercore if you're too cheap, lazy or unable to replace it yourself. I did this briefly in an old Dodge I had, then I ran into a spot where I needed heat cause it was cold and it would fog up. A tiny 12 volt palm sized heater put out just enough heat to keep the windshield cleared.
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View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)