There is solder and there is solder.
What you should use is wiping metal which is about 5 % tin & 95% lead.
This is quite flexable and is the type of material used to make up the pre WW 1 petrol tanks.
People read that tey were soldered and immediatly think of 50:50 or 70:30.
The last big use was for body filler before epoxy came on the market
