I'm partial to Yamaman's opinion on using MSR. I've done the sand/pebbles and water mix, vinegar, CLR, and even phosphoric/muriatic acid. I've had mixed results with CLR, but phosphoric/muriatic acid ALWAYS does the trick; however, I find it has many set backs...
1) It eats through everything other than plastic/rubber, which can mess up more than just the finish on your tank...
2) Since it is such a strong chemical reaction it can weaken the metal on your tank.
I think what a lot of us are failing to recognize is that it is a CHEMICAL reaction so all that rust removal is happening on an atomic level, naked to the human eye. Sure we can see the metal becoming a shiny silver and that neon yellow acid turning rust brown, but the actual removal occurs when different atoms detach and attach to one another. Just because we empty our tanks of the acid bath and give them a rinse with water and oil doesn't mean the chemicals completely stop bonding and unbonding with one another.
It's like when you grill steak a perfect medium rare; you've got a cooked outside with a pink inner perimeter and a big red center; however, once you remove that beautiful piece of meat from the grill, it's still hot and STILL cooking. The process doesn't magically suddenly stop. Let that steak sit for 15-20 minutes and you've got something closer to medium with an entirely pink inside, and that's not as juicy
What happens to the tank is that the acid keeps eating at the steel, which can weaken the overall integrity of the tank, bolt, springer, whatever your trying to restore.
3) It flash rusts faster than shit through a goose
In conclusion, YES, muriatic/phosphoric acid is awesome because it works quickly and gets the job done, but it is also hazardous and can be slightly overkill. I hope this helps guide some of your decisions!