Soda Blasting...
Pressure blasting with bicarbonate of soda, referred to as "soda blasting" will clean the aluminum valve cover and leave an as new factory finish. Then either clear satin powdercoat or a high temperature non-yellowing laquer will preserve the finish. Bead blasting will give more of a matte or slightly duller finish even after clearcoating, but could possibly work with walnut shells or plastic media. A lot of shops will bead blast with fine glass particles, but be careful. The baffles under the cover are very difficult to keep particles from enterring even when prepped with tape, etc. and will cause wear in your valve train, bearings, etc. once they release.