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No personal experience and it claims to be non-clogging ... MEANING: No need to disconnect the heater core and is (supposedly)safe for the cooling system (e.g. radiator and any other attached cooler).
However, IMO and FWIW, products of this sort a temporary patch to a problem.
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I don’t have any experience with thermal weld, but we have used similar products at the request of our customers just to see if it works.
Whether or not it works (and holds for any length of time) seems to depend on the actual source of the leak (head gasket or cracked head/damaged block) and the severity of it. We’ve tried it in at least 4 customer cars that I can recall - 1 it held long enough for them to get rid of it (a few months), 1 we never saw again and 2 of them it failed (1 due to a warped head and 1 due to pitting on the block).
You won’t be out anything but the cost to try it. If nothing else, it may buy you the time you need to plan replacing the head gaskets.