I have used walnut shell media in the past and it does a great job without leaving the sharp grit. But you still had to clean the workpiece well after the blasting. Recently saw a demonstration where the media used on aluminum castings in a foundry was soda, plain baking soda. Worked like a charm and hot water cleanup actually dissolves the remaining media. Of course I have no idea where you get baking soda in bulk. Would be interesting if anyone here has any personnel experience with soda as a blasting media.