|"You can then remove the keyplate and wash it in pure castile soap 1/3 to 1/3 ammonia and 1/3 water. This will remove all grease and clean the keyplate."|
What Ray says here is generally applicable to the other surfaces of the case as well. I have tried his formula and it seems to work providing you don't leave the solution on more than a minute or so. I'd recommend doing the back panel first to get a feel for the technique and adjust your formula according (a bit less ammonia would make for a less harsh solution). Immediately after rinsing, be sure to thoroughly dry all surfaces after, preferably with a hair dryer, to evaporate any "crevice moisture" that might cause fresh rusting. If it doesn't clean up sufficiently, repeat the process another time but probably no more.
To give your Comptometer a thoro cleaning job or perform a complete keyboard replacment, you'll want to read here what Ray Mackay advises.
Finally, a light application of oil to the keystems and oil holes on the keyplate was a cornerstone of routine machine servicing when these machines had to withstand 8-10 hours of daily pounding. In that kind of service, oiling was recommended every two weeks. For only occasional use, once every 6 months sufficed.
Before the turn of the (20th) century, Felt had made available these small oil cans for customer use, a smart move that guarded against costly repair calls.