I would buy a new core for $70 and be done with it.  Any kind of sealant
stuff would only be a temporary fix at best, and at worst could coat the
inside of your coolant system causing premature hose failures, and
inefficient radiator and heater core performance.  I have these available if
you need one.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242