I've posted about this before, but when I remove the bottom-board from a Gottlieb EM for a restoration, I carefully spray off the bottom-board with a hose and get all that dirt off. I immediately dry in the sun and with fans. Never had an issue and it gets a ton of dirt off. I can then do the small/manual cleaning of each relay, stepper, etc. as described above, but without all the dirt on the board and cloth wires.