VIBRATORY TUMBLER MEDIA??

Ground corn cob is the gentlest, especially for electrical parts. I use stainless steel pins in a rotary tumbler for all my steel/cast parts, as it won't wear out, and removes all scale and rust. I'm a reloader, so I use this to clean brass, but it does double duty now for old car parts. I'll bet some other reloaders will chime in.