check this out
Harbor freight has some fiber brushes that go on your small drop head grinder..they work well for cleaning and do not remove metal tha tis when used carefully..might be just the ticket..just be careful to not get the metal hot and warp it..then look into some of the rust treatment stuff in the body/exterior forum...
One thing not ot do is to do anything that thins the metal..like grinding a lot or sanding a lot..Sand blasting can ruin a body big time if not done by an expert..
Just my two cents..
I have tried most all of it and now do what is known to work..