In the bad old days we used acid to etch the steel to promote the crummy lacquer prime to adhere (for a while).That was so last century,now steel is cleaned and a good self etching wash primer sets the stage for a modern 2k primer.
The phosphoric acid only removed flash rust,nothing heavy so dont think it will get into the pits and dig out the worms,it will not.There is still naval jelly for the nasty bits you dont want to sand blast.
Mostly at our shop we stay away from the haz mats as much as we can,sand is still NOT on the list.
FYI the sand most small blasters need is in the concrete section at Home Depot called "arena sand"
MD(mad dog)