What's your main goal to have it clean or bone stock looking? Some tanks don't clean all that well because the fluid hasn't been changed. I have had blocks cooked that still had grease on them because of this. Tape doesn't always keep out blasting media, I found this out the hard way. What media you use will effect the surface look of the aluminum case(or other metals). Again it depends on the look you are after. If you are cleaning it to paint it the finish doesn't matter all that much.