Clear protective coating for sand cast aluminum

Hi,

I have just had some sand cast aluminum pieces made for a sculpture I am making and would like to put clear coating/sealer on them. I am not looking at anodizing or powder coating but rather a single pack lacquer or acrylic of some sort. The aluminum has been mechanically finished with a Scotch Brite mop and then cleaned with acetone ⇦ on eBay or Amazon [affil link] Flammable! . Every finish I have tried has resulted in a greying of the surface of the casting. Could someone advise me as to whether this is to be excepted because of the silicone in the alloy. I have tried lacquers, oils, wax, pva, fixative, acrylics. I just want to seal the surface to prevent further oxidization. Am I to hopeful, or do I need to bright dip first.

Thanks,