Op,
Google oxalic acid bath, Bikeforums and see what others have done to clean the rust from their bicycles. After cleaning if you want you could then use a clear coat spray to seal the frame and protect the paint...of course you may want to treat the interior of the tubes etc.
In my estimation, it may not be worth all of the effort...I might just use a good paste wax on the exterior and enjoy riding the bike, the tubing on this frame is thick enough to outlast the owner.
JM2C's, Ben