To mig weld in the whole seam would take a lot of wire or rod. Not to mention the amount of heat put onto a smaller area for a longer period isn't very good on the metal. Metal becomes fatigued from too much heat exposure. Leading in isn't nearly as hot as welding in the seam. It is about 300-400 degrees vs. about 1200 degrees soaking in to blend in a fat bead of weld onto the seam area.
Fiberglass or body filler can't hold out without cracking eventually. I've been there done that on the sail panel area.
It doesn't transfer the stress of torsional body movement on the outside skin like lead filled in.
Over time, the body filler began hairline cracks spreading all over.
Then I also had the telegraph effect where the bondo just stood out like a big band aid on the side of the quarter panel.
I know some here will say this isn't true but I tried metal bondo, regular bondo and they all looked like crap later. I just know what i've seen and I ain't gonna repeat that again. Lead for me is the only option.
