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No doubt, derusting a tank with deep rust can be an exercise in futility. I did a tank one time and one leak after another sprung up while the phosphoric acid sauce worked it's magic. Huge pain in the butt. I wound up using duct tape on the outside of the tank after brazing just caused the holes to get bigger. One way or another though you gotta derust so just keep trying until you can't see any rust inside anymore. When the metal is thin I recommend Caswell's epoxy sealer. That stuff is killer thick and he not only plugs pinholes it also reinforces the metal. It's sort of a last resort material in my view because if your tank has solid metal I'd recommend something like Por-15 sealer instead.

Okay so, in the end I used Evapo-Rust. That stuff is downright amazing. I've used the same batch to derust my tank interior, and everything that was not the frame. Stuff is seriously amazing.

I did end up sealing my tank with POR after it was dry from the rinse (I stuffed a huge rag inside to collect water deposits, then hit it with a heat gun).

Also, paint is finished and it looks amazing.