You will need to get it up to whatever heat it takes to get it apart; that may be over 700 degrees f. But then, I would clean off all the old solder. You can start with a bead blaster if you can get access to one, then with emery cloth. get it back to bare metal wherever you absolutely can. Definitely use low temp solder and stay away from acid flux; it's nothing but problems down the road.

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