The chassis was very corroded. It is steel plated with copper, and because the corrosion has come through the plating, sanding it will not restore it to as new because the copper is gone in some areas.
It could be replated with copper, but that is quite an effort to go to or an expense to have done commercially by a plating shop.
My solution was just to paint it. After some seartching I was able to find metallic (shiny) copper spray paint at a Michael's craft store. A lot of small coats were applied to the
top, bottom, and sides, until the finish was acceptable.
I also straightened it as it was a little bent (using a hammer protected by a towel).
The paint is not conductive, so I need to scrape it off wherever the tube sockets, terminal strips, and connectors connect to the chassis so that they make a good connection to ground. You can do this with a large drill bit turned by hand, and with a small file for the larger holes like those for the tube sockets.