I just got done cleaning out my tank too! It had a good quarter inch of varnish all over the inside. It was a nitemare. In my research I found you should run hard and fast from using muriatic acid. It doean't actually kill the rust, just dissolves the surface rust you see. It in fact can get into the steel and cause cracking that lets more rust in and in the end accelerates the rusting of the metal. You also have to neutralize it really well afterwards. My tank mainly had the varnish but a little rust. I dumped 3-4 galons of power purple in it for a few days and that loosened a lot of it up, then I put a gallon of acetone in it and sealed every hole. I rotated it around allowing it to soak on all sides for a few days. After that I poured everything out. I ran another gallon of clean acetone through it to make sure I rinsed all the extra crap out. After that I washed it quickly with some soap and my hose then poured in about a gallon of evapo-rust.
http://www.evapo-rust.com/ You can pick it up anywhere but I found Harbor Freight sells it by the gallon for a good price. Nice to keep a 5 gallon bucket of it to just toss things in. This stuff works slowly. I sloshed it around and let it sit on different surfaces for another day and it converted and killed most of the rust. I may still have a little more in there but at the cost of more chemicals, I'd be better with a new tank. I saved the left over evapo-rust and washed the tank out with water again. To keep it from flash rusting I think rinsed the water out with about a half gallon of gas. I saved this to run through the cheap lawn mower later this summer. With a dry tank I put a little gas in with some two stroke oil in it and sloshed it around to keep rust out while I worked on painting it. I didn't have any leaks as all my rust was just a little surface rust. For me this worked (I hope.) In your case, I'd be worried about what the muriatic acid did or left behind. A new tank sounds awfully good right about now.