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