Smoothest yet not specular metal or plastic sheets?

[skystare]

15,000 grit sandpaper, available from automotive paint supply stores, produces scratches in the 1 micron range, just large enough to disrupt specular reflection of visible light. Cheap and easy to try, just start with full gloss plastic and wet sand. Might have to go with 12,000 grit, but even that shouldn't show when projected. DIY telescope supply stores carry superfine polishing grits as well, but are thinner on the ground than auto paint shops.

 

[CWatters]

Wow. I've seen some 3000 and 4000 grit and thought that was pretty fine. Apparently some people use it to polish chisels after sharpening the edge. Never knew you could get as fine as 15,000 grit.