Drain tank, remove it, fill it with water, then solder or BRAZE (better)...............
—” fill it with water, then solder or BRAZE (better)...”
I hope you do not try that in my neighborhood.
Long ago I was a partner in a motorcycle shop.
A friend and former US Navy welder did all of our welding/ brazing work on the side.
We took a job modifying a gas tank for a customer.
Flushed with a product made just for flushing gas tanks as per instructions.
BOOM!!!
No one was seriously injured, but the volume of the tank increased by about 20%!
Best part: The customer loved it!!!