There hasn't been an epoxy developed yet that will not chip or flake off when exposed to the conditions inside the intake ports.
Many professional head shops have found this stuff to be the very best, most durable 2-part compound available:
Z-SPAR
A-788 SPLASH ZONE COMPOUND
In my personal experience best results are obtained by grinding away the cast iron "skin" which contains impurities, with a rough 80 grit tootsie roll. And clean the surface with acetone solvent or equivalent before application.
A quote from their product literature:
"Splash Zone Compound, A-788. With near magical properties, Splash Zone epoxy fits the needs of boaters, marinas, coastal home owners and race engine builders! It can be mixed and applied underwater and will cure underwater as well as in the air. This product is great for racing vehicles since it also stands up to heat, vibration and fuel on port intake manifolds."
From my lips...
