Kevin,
Oh that there was a straightforward answer to this perennial problem! The one that I have used in the past to some effect is Jenolite (the watery remover not the converter), multiple applications with a bit of encouragement from a pad of wire wool seems to do the trick.
It depends upon the severity of course, but to my mind, there is nothing better than blasting the rust with an appropriate grit, then sloshing the Jenolite over the results until all signs of the rust have disappeared.
I am less keen on the converters that claim to seal the rust. On a couple of experiments I made they seemed to work, but when flexing the test pieces a week or so afterwards, there was an unhealthy amount of rust that had been sealed under the black coatings.
If you can get hold of some phosphoric acid of a decent strength, then this is a very much cheaper way of buying the basic ingredients of most of these products.
A link to one of the 'Practical Classics' tests is
http://www.rust.co.uk/filestore/Prac...erter_Test.pdf
but I did not see the follow-up, which of course is the vital part of any test.
Good luck