IMHO JB Weld and the like are great for gluing things together and for repairing things that are broken. But they are poor at repairing things like holes and leaks that are under pressure.
For example, last year, right before a weekly cruise, I noticed that my water injection elbow on the engine exhaust was leaking. I could not get it properly repaired before my departure date. So it was either cancel the trip or effect a temporary repair. I first tried using epoxy / JB Weld to patch the leaks. That worked for 5 minutes until the pressure caused the JB Weld to get blown off the pipe. My second repair used a curved piece of metal and some hose clamps and a lot of JB weld. That repair lasted throughout my trip and allowed me to properly repaired when I returned.
In short, if you can support the JB weld with a clamp or something, it may work, if not, it wont.
Personally, I would remove the HX and take it to a radiator shop.
Deep Blue C, 2002 C&C 110
Mt. Sinai, NY
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