Radiator Repair: Soldering 'Contaminated' Cast Iron Neck

[1930-Pickup]

So far I've done a lot of work on a 1940 honeycomb radiator for my late '30 CCPU. This is my first radiator repair that I've taken to a multiple component disassembly level. One of the very last steps is to fix a leak between the upper tank and the cast iron inlet neck.

(See photo) When I removed the inlet neck there was rust at the seam, so it was easy to spot where the leak was. It actually looked like they didn't put enough solder on the joint! Cleaning the tank was fairly simple and only a smooth coat of solder was left when I finished. The iron inlet neck was another story.

After I removed the old solder and the rusty crud, I noticed that the inlet neck mating surface was discolored. My guess is it's from embedded rust and/or old chemicals.

I wire brushed it and cleaned it with MEK, but the discoloration remained. I attempted to tin the surface and the solder did stick, but it didn't want to flow. I'm guessing something is still on there.

Any ideas for cleaning the cast iron neck? Sand blast? Acid?

But wait, there's more! The tanks mating surface for the inlet neck has a wave in it. Not major, but enough of a gap to keep the solder from sealing it
up. If push comes to shove, before I solder it I'll put some small screws into it to close the seam gap...but that's my 'last-ditch bail-out' move. Any better ideas?