Safetykleen do a rust derusting solution which is truly amazing. I don't know what's in it (as the rep wouldn't say) but it was water based. He left me a barrel to try out, simply leave whatever you want derusted in it over night, next day it's clean enough to paint. I tested it out with a length of bar that had been outside for probably 20 years, it removed every trace of rust and left a pitted (but new looking) length of black bar.
If anyones got access to a workshop where a Safetykleen rep pops in, it'd be well worth asking for a trial