all acids and caustics are to be mixed that way. because they react violently with water and its a exothermic reaction it gives off fumes so do it outside and keep your head out of the fumes wear acid resistant gloves,boots,apron and face shield.

muriatic acid is also known as hydrochloric acid. its main use domestically is as acid in swimming pools. ironically the human body produces it in our stomach its what breaks down food we eat.
all concentrated acids are dangerous. yet diluted forms are harder to store. 90 percent sulphuric acid is stored in stainless steel tanks yet 80 percent of less cant be because its so strong it forms a oxide layer so quick it stops eating it away but a diluted solution can penetrate the oxide layer

the best acid to use is phosphoric acid or tannic acid as it will chemically stop the rust and change the iron oxide from to iron phosphate or iron tannate. both are commercially available as rust converter products. ranex rustbuster or fertan. no mixing needed there ready to use products. pour in and rotate so all sides are treated letting it dwell on each side for a few minutes before tipping out the excess and letting it dry for a couple of days.