I think "ally" meant aluminum or aluminium as it were.
Any kind of blasting, be it sand, glass, or any other media, will be somewhat invasive and change the texture. You don't really need this; you are simply trying to remove grease, grime and then oxidation.
I don't think that the "heat dissapation" is really much of an issue; some like the polished look, and if that is what you want, by all means...
But if you want to go back to freshly cast aluminum look, I can offer you two solutions--both of which will require you to remove the valve cover.
The first is a strong acid cleaner using "aluminum brightener". You'd have to go to a car wash supply house, or chemical supply house, because what you want is Hydroflouric Acid based. You will find some similar products over the counter at auto supply stores; in the USA, Hot Rims is one that comes to mind. Go to the shelves that house the wheel cleaners, and frankly, look for the bottle with the most severe warnings on it; that will be the strong stuff. What that does is basically eat and dissolve the dirt, grease and grime like nobody's business; then begin to attack the surface of the aluminum, making it bright. It's pretty nasty stuff. A basic wash (like baking soda) will arrest the process. There are some products out there that are not HF based; I don't know how well they work. I have experience in using the strong stuff; it was the only product that removed grime from the walls of a car wash. See my links below, any search for "aluminum brightener" will give you a better idea of the products out there.
Once you get it clean and bright, you can have the valve cover powder coated with a satin clear coat. A good powder coater will be able to help you choose. Many hot rodders and others with fancy show cars do this kind of thing all the time. Properly done and chosen, you'll never be able to tell. Or leave it as it is--once clean, it will be somewhat easier to keep clean but it will eventually go back to dull gray.
The second solution is to simply have it powder coated in a fresh-cast aluminum look. I've seen these, too--and you would have to look very, very close to determine that it isn't fresh aluminum.
Some links:
http://www.ankosales.com/aluminum-brightener.htmlhttp://www.caswellplating.com/aids/albrite.htmlhttp://www.orisonmarketing.com/corrosion/aluminator/aluminator.html