Does metal burn off? I can't wrap my mind around the statement that fusion welds are just as strong as welds with filler added. And if it doesn't burn off slightly, you must have your two panels absolutly perfect. Not one single wave that will need to be filed down. If you don't the metal is thin.
Maybe that is because I am still figuring out how to achive perfectly matched panels
And maybe it's just because we were taught to have penetration on the backside and other "myths" that only adding filler can achieve