What I don't like about weld-through primers is their name. It implies that it's safe. I mean, it's sold on the shelf, and should be welded right through, right? I don't know any welders that use a respirator on a regular basis. Even if it generally won't kill you, it's not good for you, and there's always a chance of a bad reaction.
Welding is 95% prep and 5% welding, if even that. I have friends call me all the time to weld something, and I tell them "Okay, I'll come right over when you're ready, here's the checklist..." Seems like nobody realizes the work involved when working with anything except virgin material.
-Eric