Just my $0.02:

Flat. Smooth. Clean. Has to be these.

Flat...the opposing surfaces are in contact at every point (or reasonably so). Excludes: warped, curvy, wavy, distorted by excessive torquing of fasteners, etc. Cast iron resists all of these better than aluminum or steel plate. Check in many directions with a true straight edge and 0.001" "feeler" gauge. Or against a surface plate. Or using machinists layout blue. Or even maybe using an extremely thin film of light oil.

Smooth...no nicks, scratches, gouges, pitting, etc. that connect the coolant with outside. The pressure of the hot coolant is constant when the engine is running. Even a tiny channel will eventually give the coolant a way to get through. Gaskets, sealers, etc can only do so much if the parts don't fit or aren't smooth and clean. Yes; epoxy it on and it probably won't leak, although if they are mismatched, even epoxy might give up after a while. Things that don't fit will vibrate at different frequencies and in different modes when clamped together.

Clean. If its smooth it should be clean.

oops forgot one: if its cracked somewhere, it may not be leaking at the joint. tiny crack can seep and be real hard to find...

Sorry for the long...hope it helps...good luck, I know it can be frustrating.