I use to use sponge jet products
http://www.spongejet.com/products.php
when blasting aluminum we used walnut.
for paint straight up soda.
for paint w/rust, tar or bondo we used a soda/aluminum oxide mixture.
I know alot of ppl don't like using any type of aluminum oxide on sheet metal because of the warping. However, if you have the right equipment ($$$$$$$) like something offered by sponge jet which allows you to dial in your velocity/media ratio's it's the best for removing unwanted material safely.