I'll agree that soda and plastic are not agressive enough to remove rust issues, but sand introduces salts into the mix, not a good combination pounding against bare steel. One of the members on Chevy Talk was having issues with spots of rust having formed under his epoxy paint (found in subsequent body work) and come to find out, he blasted with sand.
Here is a link to the threads by Porchdog on sandblasting, he makes some recommendations for media that is just as economical as the sand you purchased without having the ill-effects of salts on your bare steel.
Porchdog's sandblast posts
