I also did some DIY with an old junk tweeter. Apparently at some point, someone has tried painting it with something funky and it was cracking and all caked up on the surround. It wasn't very loud, sounded muffled and couldn't carry a tune with a bucket.
At first I tried cleaning out the old ferrofluid and adding fresh. BTW even after 30 years the original ferrofluid was still in there. Changing it didn't do anything, except maybe make it quieter.
Then I took the dome assembly and cleaned all that gook off with nail polish remover. I was super careful, used about 50 cotton swabs but got all that gook off and cleaned out the surround, (it must've had a couple mil of buildup).
By that point the dome was essentially just fabric. I could blow through it butt could see through it and it was extremely supple. Put it all back together and it worked and sounded smooth, but very very quiet.
Then I took it back apart and carefully coated the dome with a base coat and then a top coat for nail polish. I figured fabric is organic and pliable so are human nails, so what the heck why not try? Application was easy enough, it went on smooth and didn't seem very thick. I let it dry in between coats. It's still pliable, but certainly stiffer than it was. I put it all back together and it actually works again! It's fairly loud and smooth.
I didn't take any measurements, nor done any critical listening but it does work and what was a garbage tweeter is now usable and I've got it in a project center channel I'm tinkering with.
Here is what the tweeter is supposed to look like, (I've got several that are mint):
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Here's what the junky one looked like:
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Here's what it looks like after I got done:
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Viola!