[Phlogiston]

Here is the thing about painting a chassis that was originally unpainted.

First, it is hard to do. If you can remove all of the components, then you can do a paint job like it should be done. It is really hard to get a good job when you are just masking-off transformers, tubes sockets and such. But if you are going to remove all of the components, you might as well have the chassis replated (not to be confused with chroming though that may be the plating on some chassis)

Now, if restoration to a factory appearance is not important, then you are probably not going to the effort of stuffing caps and such. Then painting the chassis might be acceptable but there is really no reason to go to all of the work on the underside if the chassis is to be painted since just about anybody is going to be able to tell that it is not original.

When painting a rusty chassis care should be given to ground connections that rust or paint might insulate.