$.02: The surface of the cover is very smooth but not polished. Don't blast it as the texture will roughen the finish. Typically it is just oxidized and lightly stained, blasting isn't needed. Mine is just degreased and given a coat of WD40. I don't like the polished shiny thing, that''s too far from original. I like the patina. The manifold cleans up perfectly with just degreaser. I don't have a better protectant than just WD once in a while. It even looks good on the injection lines. Natural beauty of original surfaces.
If you degrease and don't coat with WD, everything shows the oxidation. JMO...