For what it's worth, I treated the gas tank in my dad's 1948 Diamond T last summer with the POR-15 tank restoration kit.
It took the better part of 3 days to complete the process. The tank had a lot of scale in it so I spent most of that time swishing first the Marine Clean, then water in the tank, then flushing and repeating the process. The same went for the Metal Ready solution. When I was done, the inside of the tank looked fantastic. To make sure it was bone dry before I coated the tank with the sealant, we rigged up a heat light and a hair dryer on low temp to warm the tank and circulate air through it. Worked like a charm.
I wouldn't hesitate to use the POR-15 kit again if I ever have to do another tank. :thumbsup:
Good luck!