It is typically used only between the metal of a lap joint that is then spot or plug welded.
Supposed to help prevent rust in that lap, and while it does burn away at the weld (as will any coating) it protects the metal in the immediate area. The idea being it is less heat sensitive than other primers and will remain and protect closer to a weld.
It is also an excellent way to contaminate a weld, so "weld through" is misleading.
If doing plug welds it's a good idea to scrape/clean it out of the hole you're welding, and a little more outside of that if you can.
When I worked at a vintage car restoration shop we used it often, thought it necessary, and we despised it too. I made up a spoof paper label and wrapped it around the spray can. It read something like "Spray on Weld Contaminator... best when used liberally".
For exposed metal there are lots and lots of better primer choices.
Grigg
