I don't know how bad your valve covers are but I've blasted a lot of stuff with different media and there is no "blasting" that will polish. Personally I gave up on polished aluminum 40 years ago. If it hasn't been anodized then its just going to oxidize and need periodic polishing - which can be a PITA to do on many parts. All my aluminum parts get the casting edges blended, blasted with aluminum oxide, washed with aluminum etch, primed with yellow zinc primer then painted - usually "blasted aluminum".

But back to your valve covers ... if they're not all banged up or have deep gouges (in which case polishing is pointless) I'd first wash them down with an aluminum cleaner which will be slightly acidic to remove oxidation. They will likely turn a little grey. Then I'd go at them with the finest scotchbrite or 0000 steel wool. Then I'd hit them with an aluminum polishing compound. And if you want to spend the bucks you could buy some different size buffers that fit in your drill to get in the tighter areas.

But I like paint !! The only aluminum not painted are the alternator, pulleys and water neck.