Here you find a full explanation of how it works.
http://myweb.tiscali.co.uk/andyspatch/rust.htm
I for one can guarantee you that it works, seen it proven on paper and happen in front of my own eyes
You have to be aware that rust is a binding between oxygen (from air) and iron itself. Most of the rust will be detached from the iron if you remove it, so no matter what method you use, de-rusting any part will leave you with less weight and strength that before the rusting started.
So yes you have a clean peace of iron, but if you really need the original strength, you should prevent rusting beforehand.
