I used Copper Coat,

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=KW_401612_0006406066

I sprayed it on both sides, waited a few minutes, sprayed it on again, and set it down. The stuff is REALLY sticky, I'm thinking it'll seal pretty well.

You're much more patient than me. It would kill me to assemble the whole thing just to paint, and then tear-down/reassemble. It seems so much easier to just mask and paint, but to each their own. I'm sure both ways have positives/negatives.

On another note, are you replacing the hoses from the water pump to the heater? If so, with what? I can't seem to find any suitable rubber hose at any of my local FLAPS...