Acid washing of sand is done to displace any cations (e.g. K, Ca, Mg, Fe ions) off of the sand's cation exchange sites. This is followed by rinsing with deionized water to get rid of the Cl-. At that point you have clean sand and have complete control of the nutrition that the plants will receive. To do this you could fill a container 1/2 way with sand and then add the acid solution and mix it together. After incubating you can pour off the excess solution, rinse with more HCl, and then begin rinsing with water. I imagine that HCl is chosen simply because it is readily available, but that is speculation on my part.