Most epoxy compounds are "exothermic"- - - - -they generate heat chemically as they cure. Thicker layers retain more of the heat of curing instead of radiating it out into the surrounding material. A layer no more than 1/4" thick should be all that's needed for a good bond and enough strength to resist the pressure inside a stovebolt cooling system. Press a piece of 40 grit sandpaper against the patch just before it cures fully to mimic the texture of the surrounding cast iron.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!