I cleaned mine carefully with some fine grey scotch Brite. The manifold is magnesium, your not going to be able to finish it like the factory did. After getting mine as clean as possible I painted. And baked on VHT brake caliper satin black paint. I think I lightly soda blast all the white deposits off before the paint.
Page 12 of my thread I did the manifold.
"Barn find" 1986 911 turbo