Also check out my thread
(rust removal via chemical means)
I have been using a feed grade molasses and water mixture. Make a soak tank and drop the parts in for about a week, rinse with trisodium phosphate and water, and let air dry, then paint. Some stuff like stamped steel it works great. Body bolts like sems screws and nuts it tends to eat up. I soaked some of mine and ruined them. Had to go to the junkyard for more. Must be a different steel content or something. This stuff stinks pretty bad though. My soak tank is behind my shop. Feed molasses by the gallon at a local farm supply store is about $10