Over the decades, there is all sorts of chemistry happening with the original lube, heat, internal off-gassing, external smoke, dust, skin, etc. Add to that the cleaning products used over the same decades. The issue with spray cleaning them is that the lube removed has to be replaced in some way, both for the actuator and the contacts.
I have been maintaining a few Dualitys and a 9K for almost two decades and have found the Caig ProGold to be the best cleaner/conditioner in one over time. I was just mapping the 9K yesterday, and many of the switches cleaned in the past stay clean. Especially the ones that get used regularly.
Biggest best thing for switches is to be used regularly.
Others are a few pushes away from usable. When I started cleaning switches I would have to clean 4-7 switches per module, but yesterday's mapping was 1-2. And hardly any harmonics component on the bad switches which was out of the ordinary. Just fuzz and crunch. Some just need another Pro injection.
I was never a fan of ultrasonic dipping, but it certainly worked well with SSL 4K's. It was that once you stripped all the factory lube out, you had to keep with a regular dipping/lubing schedule or it would get noisy real quick. I think it is a disaster on 80 series Neves.
I would never use emory cloth on an audio switch contact, and I can't get to the contacts in a 6PDT e-switch, ITT Schadow, or Neve rotary anyway.
Mike