I've been going around with this for some time now, having ugly areas within some frosted glass windows. What I just discovered though is that using the Noise Filter in this case seems to be what's killing the effect.

I'm using the built-in Dielectric preset for Frosted Glass as my starting point. Works great on sconces and the like. Thought I had an ah-HA moment with these windows when I discovered the 4 polys in question were just single sided, so went and gave them some thickness. When just that didn't do the trick, I re-surfaced so one side is frosted & the other is smooth but still using the same basic dielectric. Even stupidly high render & GI settings still weren't doing it:

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As I watch the render progress, I noticed that after the blue phase of the render passes the window area, it's always looking pretty good. So as a one last thing to check before posting a question, I tried w/o noise filter:

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Of course now everything else is a bit noisier.

So I guess the question is now, when we gonna get some options & selectablility with Noise Filter?

In the mean time, any suggestions? A different material or technique?