I use an air jitterbug when necessary - no danger of shock there.
But on those valve covers I usually use a drum sander, but gotta keep the piece moving to avoid ripples.
BTW, don't be surprised if you find "pits", which are really pores within the cast aluminum - you'll never get these out since new ones will appear as you keep sanding. Nothing you can do about it unless you plate them.
And, for you and all the newbies - I sure hope you are wearing at least a dust mask, preferably a respirator, while sanding and polishing. Ingestion of aluminum has been linked to nasty stuff, including the onset of alzheimer's.
We should stress safety more in here.