The valve covers are cast aluminum and the original finish is not polished but a dull, natural aluminum. With age, the aluminum corrodes and that is most probably the finish you are dealing with. When I rebuild engines, I take the blocks and machine parts to a machine shop that cleans the parts, engine block and such, in their cleaner tanks. That is how I get my aluminum back to the natural finish. Soda / hydro blasting will provide a similar finish. Media blasting will pit the surface. If you polish the surface, the aluminum will loose its dull, natural finish and become chrome bright, silver shiny. The more you polish, the more shiny mirror finish you will get.