I was helping out a chap that collects military vehicles from the 70s. A
coolant reservoir had developed a pin prick rust hole fairly near a
seam. Initial inspection made me think it was basically a steel
cylinder construction tinned after the joints and pipes had been
attached. The tinning had corroded to allow some light rusting over the
surface.

I chose to braze the hole shut. The interesting thing, to me was the
adjacent joint was some sort of hard solder. It would have been far
tooexpensive to be silver solder. Anyway I sealed the hole and the
adjacent solder melted a bit but no damage.

I now need to address the surface rusting and was thinking of using
hydrochloric acid and then wondering about re tinning, any suggestions
for what to use? On another rusty tank I previously used zinc chloride
flux and an old lead based solder, a heat gun and wiped it on but it
was a bit patchy.