Please Explain Weld Thru Primer

I have been using it for years and I have said before I have put a lot of truck box sides on over the years using a wire feed welder and replaced the box sides on many trucks 3 or 4 times in a 5 year period and when I take them apart the weld through primer has still been good even around the welds.
I am not the best body man out here and if there is something better than please let me know.