A Dremel-style rotary tool and fine grinding discs are your friend. Use heavy tape to protect the leatherette around the rivets, put some good music on the radio, and TAKE YOUR TIME.
These old boxes will clean up externally fairly well. If there is a B setting the shutter will remain open enough to get the inside with a q tip. I have been able to gently coax those viewfinder lenses off some models then press them back after getting some of the dust out. Unless they have been lucky enough to be stored in a dry place rust is inevitable and the silvery stuff in the viewfinders has flaked off. Straps are rarely intact if there at all. Cameras like these except some rarer makes are plentiful so finding a better specimen might be the way to go. My priority is to take pictures with them so I forgive their looks. The simple shutter mechanism almost always works.