Anytime you solder or braze, the base metal must be polished clean and the solder free of oxidation. The idea of having heat shrink tubing and solder in the same body is good in theory, but the solder inside the tubing will oxidize and will not flow without flux paste. I tried with and without the flux paste, then opened the tube to check adhesion and it wasn't even close to good adhesion. It dissolved.
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