Muriatic acid is nasty shit and I have never used it for fuel tank rust.
it does have other uses.
I take a hand full of 1/4 nuts ( or gravel/stones) and put them in the tank and shake the tank vigorously for as long as you can, the longer the better.
if you don't have varnish type of crud in the tank, just rust, you only need a bit of your favorite flavor of liquid soap added.
When you figure you are done, wash it out good and let it dry in the sun.
You will have removed the loose rust, any remaining fine debris will be caught by the filter.
if you keep fuel in the tank it will not make new rust.
works for me.
