I really don't like the so called weldable primers (or any paint) because I am not convinced that they do not affect weld quality. Also someone has to breath the fumes.
However, we do use them from time to time at the customer's request. We usually use a mist coat of zinc based primer and try to keep it to 1 mil or less. On small jobs, we have used either cold-galv or aluminum paint spray bombs.
It seems that whenever we use those we can get decent looking welds and not too many wedability complaints.
I've heard of paints sold for that purpose, such as "Bloxide", but have never tried them.
CHGuilford