All--
Jeffrey's admonition re not using sandpaper (or crocus cloth or emery paper, for that matter) on the keyboard contacts should be heeded. There is a contact burnishing tool in the usual Teletype tool kit for this purpose. See the tool listing on RTTY.com. The keyboard contacts are made from a special silver-bearing alloy and can be irreparably damaged if mistreated. (The contacts of a telephone No. 5 crossbar switch is a similar case in point.)
Use of brown paper such as that from a brown paper bag with some contact cleaner or rubbing alcohol can be used. Fold a small strip of the paper in half and saturate it with cleaner. Place the folded strip between the contacts and pull it through while squeezing the contacts together with your fingers. Several tries may be needed to clean the contacts adequately. The wood fibers in the brown paper act as a wiping mechanism.
An old mentor of mine (long gone, unfortunately) introduced me to the concept of Sealing (or Wetting) current. This is the amount of minimum current needed to make sure contacts carry the signal through them adequately. This notion applies to toggle switches, relay contacts, push button switches, etc. (Try Googling Sealing Current; you might find some useful information there.) It is not clear to me what sort of circuit arrangement is being used in this situation, but the recommendations of using a standard 60 mA loop with a compliance voltage of 80 - 120 volts should do the trick. I used this set-up and the cleaning procedure above on all my mentor's many wire service machines with success.
Good luck!
George, W6BN