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Perplexing problem... would love an explanation.
There is a very paint like metallic silver coating on various copper bits. Its wipes off easily to the touch, floats on the surface in water, and may also be leaving a silverish hard coating on the plates. You can see this in the last picture. Scrubbing hard with a stainless brush seems to remove this coating, but Oxiclean doesn't do much.
This doesn't happen all the time. Only for "beer" mashes (malted grains) not for rum/sugars and oddly, apparently not for rye. Low wines of beer mashes are ok too.
The scrubbers are located in a glass column section after the dephlegmator and before the condenser.
Copper reacts to certain chemicals obviously, but not exactly like what I'm seeing here as far as I can tell.