A Related Problem, searching for a workable solution..
I just adapted a '87 Volvo 760 Turbo Intercooler.
It has a leaking heater core- about a teaspoon every day.

The catch: It has 'climate control' therefore to access the core
is about 16 HOURS of labor. I have been quoted $800-$1200 !!!

I tried a commercial radiator sealer today, will report when
the result is known.

If this doesn't work, has anyone tried cleaning out the inside
of the tubes and coating the inside with epoxy paint, or other type
of sealant that will coat the inside of the core, sealing pinholes?

I have nothing to lose, so anything other than 16hrs of pain
sounds like an option worth exploring.

(If only THIS engine was in my Westy!)

Paul '87 Westy, '87 Volvo wasserleaker