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I picked up an extra case, and it has some (a lot) of battery acid "damage"..

I know it can be blasted off, removing like 0.01mm of surface

what is the best type of media to use? I am doing this at home, I have a small blast setup.

Carbide? Walnut shell?

Hey Isaak,
The carbide may be too agressive and the walnut shell may not be agressive enough.
See if you can find used silica bead sand for the cases.
New silica bead is ok but you would have to be little more careful.
I had a friend that worked at an air damper manufacturer that blasted the louvers and air dampers before painting them.
They would use the media 2-3 times which would dull the sharp edges of the particles then discard it. The discarded media was perfect for engine cases on a BSA, Norton, and BMW R-60 that I restored. It will leave the cases with a similar patina to that which they had when new. It will also remove a fair amount of damage as long as it isn't deeply pitted.
Look in the phone book and see if you can find someone who actually does blasting or a metal manufacturer that needs to clean up their product before final finishing. If they use silica bead media they would probably let you have a couple of pounds of their used media.
Hope this helps,
Dean

thanks for the tip!

I got another reply privately -- to use coarse grade walnut and just do it for a long time.

Sounds like you are on the same track.