It did not even occur to me that your were simply masking the welds so you could paint. I took a different approach and sprayed the seam with copper infused weld through primer.They are stickers that are placed where my welds are going. Lots of extra work but I decided it makes sense to have paint anywhere I can.
I could not replicate what Ford did with their spot weld through seam sealer. I figured anything was better than leaving the metal bare. I have had my eyes opened as I have replaced panels and found rust between the layers of spot welded metal, for example the tail light panel.