[RickM]

For blasting I'd suggest plastic bead. There are many variations of this media and it can be done without harm to your galv tub...if it's coated in the first place. Soda blasting has also been suggested on this board. I don't have any experience with it but if you can find a shop to use Soda you may want to compare.

Regarding primer, that's a tough one as primer is not designed to be a toap coat and in some instance will allow moisture to penetrate.