WI,
I use the weld thru primer where the actual welds go. I haven't gotten that far in the panel prep yet, but I'll grind off the EDP that comes on the replacement parts, coat that area with the weld through primer, then mark and drill the holes for the plug welds. My hope is that the weld through primer won't completely burn away and offer some protection surrounding the weld site. I use the SEM Copperweld and am pleased with how it works. OBTW, you have to coat both pieces that will be welded, per the manufacturer. Marking and drilling first also works. I'll look into that product for the cavity.
