The pros and collectors over at the Wagner and Griswold website use vinegar to remove small amounts of rust on cast iron that isn't too rusty. However, they recommend a 50-50 mix of vinegar and water. Here's a link to their cleaning page - check out the paragraph titled RUST.
http://www.wag-society.org/Electrolysis/lye.php
In my discussions in the forum, they recommend soaking the piece in 30 minute intervals. Soak it, rinse, scrub, and check it, then repeat if needed.
Generally speaking, however, they tend to use the electrolysis method a lot more often. Then again, some of these collectors have a couple hundred pieces in their collections and clean/strip a dozen or so pieces at a time.