One of the youtube “Restoration of Lionel Prewar Trains” video series mentions something about repairing the pits left behind by corrosion on the sheet metal parts of the cars. However, there is nothing else about what material is used to patch up such pits. Automotive body filler, maybe?
Or perhaps solder? Nearly pure tin solder is available, with about 5 percent antimony. (Pure tin is subject to “tin pest” at low temperatures.) I have used it for cosmetic repairs, because it remains shiny, unlike tin-lead solder, but not on toy trains.
used JB Weld…main;y because I have it on the bench.