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Chuck,
I think those screens and the aluminum ones that came with the Advent Videobeam projectors with the external curved screens in the 70's had a micro etched surface finish that would be polished flat if one touched them even gently with a cleaning cloth. The only way to clean these screens was with a type of soap gently sprayed on and a clear, warm water spray rinse. Fantasic, the spray cleaner works great (no physical brushing or rubbing) and dissolves smoke resins that get on this equipment in Bars and taverns. The same treatment works great on the front surface mirrors in the projection schmidt optics (provided the mirror is overcoated with vacuum deposited silica).
You may be able to revive your screen with very delicate glass bead or sand blasting. I think the Philco screen is aluminum overcoated stainless steel formed sheet
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