If you’re talking Neumann type center terminated capsules, you can try brushing off the edge and just inside where there is no gold, possibly the center electrode area, with a camel hair brush like you use for lens cleaning…some people use camel hair paint brushes. You probably don’t even have to touch the gold area to get it to stop shorting out, or at least it’s worked anytime I’ve left it at that.
You can probably get away cleaning the gold on newer capsules but as ruud says, the gold can be very easily disturbed.
I have used an eye dropper, distilled water, and wicked it up with thick water coloring paper…not making contact with the diaphragm.
Dan’s method is interesting but you really don’t want to have any water get behind the center post or in the holes of the mounting rings, so it feels a little helter skelter. I would be curious to rig something that creates a very small pinhole type stream that I could stop and start…I can think of some ways to approach that, but I haven’t done it.
I have heard of other things but I don’t really know the validity, safety, etc. Some people would spray the mic diaphragm with freon or other electronics cleaners. I have heard of people using ultrasonic cleaning approaches as well, I think along with taking the capsule down to subparts level depending on capsule.
I would keep in mind that you can always buy a cheap capsule to work with first.