Luke, if you use the search function you can find several threads about this, but basically you're supposed to follow the muriatic acid wash with a phosphoric acid wash, it reacts with the newly exposed metal somehow and produces a more rust-resistant layer. Some people have used Coca-Cola (which contains phosphoric, citric, and carbonic acids) instead of the harder to find and more expensive phosphoric acid. If you use Coke, you need to make sure you've rinsed out the sugary residue... you can use denatured alcohol for this (it will also dissolve dried-up gas varnish).
I don't know if this is a good idea or not, but it might be cheaper and easier to use 91% isopropyl alcohol instead of denatured, you can get it at most drug stores. 72% is probably no good, it's 28% water and the leftover water can start re-rusting your tank, but in my experience the 91% evaporates pretty completely. Never tried it in a tank, though.