rust protection
 

I'm having my car soda blasted when all the welding is done what is the very best treatment to have done to the tub to keep rust away

we never soda blast our steel bodied cars .just about any other media is better to use over soda .

as soon as you get the car out of the blasting booth you clean and prep the body . you then read the TDS of the epoxy primer you want to use . the TDS will tell you what grit sand paper they(paint manufacturer ) wants you to sand the body with so there primer will stick .
we use the epoxy primer mixed as a sealer . we do this not to add mil's or primer but just to add rust protection over the bare body . you would do your body work right over the epoxy sealer .

many think doing things like soda blasting of other ultra fine blasting media will not take off the galvanization coating on the body . wrong it takes it off . it's so thin just sanding it with 180 grit removes it . this is what the epoxy replaces and one reason why you use it .
you want to keep the mil's of epoxy thin the thinking more product is best is wrong.

I have heard nightmares about paint adhesion problems after soda blasting.

you just have to clean the crap out of the car and when you think it's clean you still better do it another few times . other then that you can soda blast a steel bodied car .
there is no reason to and in fact there are so many better medias to use why would you even think about doing it ?