Patching holes in a heater core

I put a nice little (3/16") hole in my heater core on 1 side and the seam at the top is seperated. What's a good method to fill in the hole and to reseal the seam? JB Weld? Solder? Any tips appreciated

Solder it.

Soldering is the only way....if it is leaking in the fin area, you will have a rough time. If not impossible..........

I have very very easy access to where the problems are. The seam on top is open, so I can solder that shut, but the hole I put in is accessible from the center easily.. I wasn't sure if I should pinch the edges of the hole closed, then solder, or if I should just solder on another small piece of tin over it so fluid can still flow in that little area. I'm assuming the solder will hold up to the pressure?

Your core was assembled with solder. I like the idea of laying a small piece of brass over the hole as opposed to pinching it shut.

I can't get the solder to stick to the heater at all. I've used a degreaser/waxer on it. Dried that, applied lots of heat to it, put on flux, and it still won't even come close to sticking.

I'll give the cleaning a whirl and double check the type of solder I'm using. If not I'll go to the radiator shop. This thing probably hasn't had antifreeze in it since the Ice Age