.3% oxalic acid solution. it's found in hardware stores as "wood bleach", but can also be found online. basically dilute it to that particular solution and soak the frame (clean and devoid of any wax) for several days, lightly rubbing off the rust every so often. once the rust is gone, frame saver and wax the frame.

where I bought my oxalic acid: http://www.van****staxidermy.com/Default.aspx