Hi Dave,

My usual way of doing this (not sure if it is the tradiational “correct” way!) is to run round the solder joint with a blowtorch on a low heat and wire brush off as much solder as possible using just enough heat to melt the solder and working an inch or so at a time. Once as much as possible is off then I heat a wide area and gently pull the joint apart in the shear direction perhaps 1/8″ then move on around. Work round the tank and once you have been round a few times you should have worked the tank cap up far enough to seperate it. The key is to use just enough heat as too much will warp the steel sheet.

The above has worked for me on a few tanks and radiators that I’ve had to repair.

Cheers,
Richard

Oh yes, perhaps I should have mentioned the important bit… You can either put the tank on a radiator for a few days to evaporate any volatiles, or usually I purge the tank with argon or CO2 (mig/tig gas) and keep a steady flow into the tank as you are working with the torch.

I’ve had tanks flash even after hot water washes etc so I don’t use that method any more.

Always do it with the tank cap off too so if it does flash the pressure build up is very limited.