Dear God. No. Maybe on an older technology vehicle, but the seals and gaskets in today's motor vehicles are far, far too sensitive to be using Bar's leaks or anything of the sort!!! NO!Depending on the repair longevity you seek, a bottle of radiator stop leak is an inexpensive alternative. Years ago I used the stuff on an aging car to cure a head gasket leak and it held fast for years. A heater core leak would likely hold for much, much longer. Just a thought.