"Milk Stone Remover" (MSR) - available at Farm and Fleet, Tractor Supply, places like that.
It's about 70 percent phosphoric acid, dyed orange for safety - mix it down by half with water for use.
It's actually a dairy product, farmers use it to clear calcium dpeostis from their stainless pipework.
My rust remover of choice, I keep a big cooler full and process nearly all my parts through it.
Hell on rust, won't touch chrome, fairly safe to use.
Absoulutely great for de-funkifying tanks, gets every nook and cranny.
bill