I know this has been touched on in several other threads and posts, but, as these are getting somewhat long in the tooth, I'm going to present what has not been talked about:  Rust issues inside access panels due to the noise-deadening insulations typewriter companies used, along with the various glues they cemented in those insulations.  Right now, I am working on several Underwood SX-150 machines--all with the same problem:  Rust on the insides of the front and back plates, and on the paper tables.  They used some stuff that, over the years, that was either corrosive in its own nature, or attracted and held water in those spots.  Whatever the dynamics--the end result is when the insulation is removed, what is underneath is horribly rusted.  The SX-150 is one of the Greats--Best Underwood ever designed.  But for some reason, one of its weak points happens to be a problem with rust under its noise-deadening "stuff."

What I'm doing at the present is removing all this insulation, wire-brushing and sanding the bare metal underneath, rust-curing the metal, spraying the metal with several coats of primer, and spraying on this stuff called 3M Rubberized coating.  I am hoping this deadens the sound a bit while providing a more metal-friendly way of doing so.