[Rich in Tucson]

Whatever you use to clean, derust, or coat a tank it might be worth your time to rig a motor and drive to make a rotisserie that automatically inverts the tank end-over-end (or whatever) while you sleep or do something else. I have seen a belt-driven tire (on a fixed wheel and hub) to which a gas tank can be strapped with a belt drive and a gear driven one. Sharp crushed glass is one of the mechanical means I have seen being used to clean internal tank surfaces (newer gas tanks, not Model A). The volume of solution required with rotation may be much lower, but then you may run into the limits of the chemical conversion capacity of whatever you are using.

I have gotten good results with new Evaporust on some small rusted things however its capacity is very low and because it is made of organic chemicals reuse is usually out of the question since it is degraded by bacteria very quickly and seems to turn acidic which can further damage what you are trying to preserve if you trade longer time in the solution for reduced capacity.