The message
from Dave contains these words:

I think the tank should be filled with water - again afaik.

No - you can't weld to it when it's full of water. For one thing
there'll be water leaking out of the hole you're trying to weld up and
even if it weren't, on such thin sheet it'd be a bugger to weld to
something chilled by the water.

The trick is to empty the tank, fill it with water then empty it again.

--
Skipweasel
Pay no attention to that man behind the curtain.