I am rehabing my 1992 PDG 5 which is in similar condition. There is some surface rust on the underside of the top tube and the bottom bracket shell. I bought some touch up paint hit the spots under the tube after taking steel wool to the spots. I also got a can of spray paint (automotive) and masked off the BB shell so I could cover up the rusted spots around the cable guides. The white is not a perfect match, but it will be hidden. I also sprayed the rear dropouts as they were a little rough. I plan on riding this bike so I am not worried about it having perfect visuals.

If you plan to ride it, clean up the rust spots and cover them with something and enjoy.