The first thing to do with any steel fram bike is to take the bike apart. If the inside of the frame is in good shape, spray the inside with WD-40. There may be better rust preventers, but WD-40 is an old-time trick for preserving the interior of a frame.

If the inside frame shows some signs of rust, you can spray with rust converter that you can get at a hardware store. It won't restore the strength of the frame, but it will stop the rust cancer.