Most likely it's acrylic plastic. Super smooth out of the mold (not at all unlike gel coat on your hull) but with time the surface develops pits. Mother Nature and the Sun are merciless over time and you will be hard pressed to get any kind of smoothness back. You certainly didn't help it but you tried. You could try sanding with ever increasing fineness of sandpaper but even then, acrylic doesn't act like gel coat. Painting might help some :| but still......
Your best bet for a good looking dash is to replace the old with new.
Good luck.
Edit: You're right Red, they may just be starboard.
The Gallery shows an 87 and it is indeed grey plus when you look close, there are no cracked corners. Unheard of on just about any Aqua dash that old.