There's stuff called "Naval Jelly," it's pretty potent phosphoric acid, but not to the point that it'll burn you if you get some on your hands (at least not right away, don't dunk your hands in the stuff thinking it's safe). It's normally about the consistency of yogurt, but there's a sprayable liquid form too. worked great on my spree tank.
I just baked my tank to get the water out. I turned the stove to just above 200 degrees and let it sit for fifteen minutes. the fresh metal will rust quickly this way, though. Nothing serious, though.