Don't use an emery board to clean contacts on electromechanical pinballs.
If you do you'll end up with some switches that won't work because the emery
board will leave grit in the switch. You can solve this by blowing on the
switch after you file it. The preferred method is to clean the contacts
using a flexstone or point file, both of which little or no grit.
Note that you should NEVER file a gold-plated switch on a solid-state
pinball. Almost all switches on solid-state machines are gold-plated.