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.

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.