Foggy lens element rescue

[valid]

Hey guys,

I salvaged an old anamorphic projector lens, and there's an issue with the rear element (specifically, its outer surface).

It's foggy (almost like ground glass) - but it also has some tiny splotches which are clear. Based on the shape of the splotches, I figured it was somehow fungus related, so I gave the lens element a short vinegar bath, and then also some stronger fungicide, but it didn't change anything. I also swabbed the surface with acetone (I figured I can at least take off some of the coating) but again, nothing happened. This thing seems to be bulletproof... The other side of the element, as well as its cementing, seem to be fine.

I attached some pictures. Any ideas how to restore this? I am happy to strip away the coating or any other drastic measures, as at the moment this element is unusable.

 

CLEAR SIDE:

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FOGGY SIDE: