I tried this stuff recently, on the recommendation of someone in the motorcycle community:
(http://www.evaporust.ca/images/ER_gallonjug_new.jpg)
Pros:- easier to handle than phosphoric acid;
- claims to be biodegradable;
- works as advertised -- we used it to clean up the inside of a neglected motorcycle fuel tank. Before... crusted with surface rust. Filled the tank with the stuff, and allowed it to soak for ~24hrs. Noticeable results at about the 10 hour mark. After 24hrs... slightly pitted, but clean grey metal.
Cons:- local availability is via Canadian Tire, 1L bottles only (mfgr also provides it in 4L jugs).