Just got this how to from a person off of ebay. Don't know if it works but I guess its worth a try!
Cleaning Cast Iron Rust.
Don't know if this will be consider helpful or not but since these tend to sell better in good condition I thought I'd offer a tip on getting it cleaned fairly easily. :o)
Use olive oil and a little pile of salt! Heat the pan, pour in the oil then dump in a small pile of salt. Turn the heat off once the pan is quite warm and use the salt to scrub the rust out - and there you have it! Works on the inside and the outside as well but I do the outside of pans over the sink. The more rust the more work of course but it's a great way to salvage an old cast iron pan that could be worth considerable money. ;o) I've saved many pans from the garbage with this trick I learned from a chef, and made a lot of money off them once they were clean where nobody would touch them with that rust!