I believe the results have a lot to do with the product used. I used EuroSpray Weld Thru Primer Copper on my build, it was what the local automotive paint store had. On test pieces I did not scratch through the primer and welded normally with good results. On the car, I had some spitting and spattering on the beginning of new plug welds. I tried scratching through the primer thinking it would allow a better arc to strike, but it was no better. I think it had to do more with 50 year old metal being involved.
Another builder here, 7T02S, used Cobre Weld Thru Primer with good success. I believe he welded through the primer. Based upon his and my limited experience, buy the best weld thru primer you can get.

 

I use an end mill in a drill to clean the weld thru from my plug welds. You want to minimize the contamination in the weld as much as possible.

 

I was using the Eastwood weld thru primer and when you initially start the weld, there is some sputtering.
As you progress, it returns to normal, which I attributed to the heat burning the primer off.
I stopped applying it before welding, and would brush it into the seam after welding.
Since the paint has such a low viscosity, it flows everywhere and it won't get burned off.

 

Weld through primer is one of the most misunderstood materials in auto restoration. It’s meant to be used for blind joints that aren’t sealed. It protects the backside of the open weld and any place you won’t be able to get paint or a sealer after the parts are welded.

For example in the OP’s case it should only be used on the blind faces (in the “sandwich” of the metal) and not the outer surfaces of the joint. The welds should be short, hot blasts. The welds cause a reaction with the zinc (or whatever metal is used in the primer) which makes the backside of the weld resistant to corrosion.

 

I've been MIG welding on cars over twenty years now. Weld thru primer is something I tried for a while with limited success. Like others have said, the weld will sputter at first but then even out. However by that time, the weld pool is hot enough you could weld through practically anything - rust, paint, etc. Also, unfortunately by the time you get the primer burnt off you're heating the metal up and there is going to be a lot of warping if you continue welding. That is the biggest thing you want to avoid because it increases your time and cost in every aspect. Now I make sure all of my welds are only against clean bare metal. If I'm concerned about the metal corroding after I weld it then I use caulk to keep the water out. Metal will not rust without water. Look at the backs of dashboards and other areas inside Mustangs which never got any primer but rarely are extensively rusted.

david

 

I have a video by Craig Hopkins who said to use Rubber-Seal RS-512 etch weld thru primer. He did say that if you don't do the welding that day, you may have more trouble welding. It should be between parts, or on the inside of a cavity you can't easily get to. He started the weld on the primer.

 

I gave up on weld thru primer, its messy and by the time you've burned through the primer you've typically over heated the panel.

 

I don't use it anymore either. For vintage car restoration I don't think it's necessary. Our cars will never be driven on salt covered roads. For the collision industry yes I would use it on lap joints. Weld your panels in without it and follow up with cavity wax after paint for protection against moisture.