Laminated Metal?

Picked up a decent running 99 GMC Jimmy 2 door for the winter, as we are filling rusted holes on the rocker panels with new metal, it looks like its some kind of laminated metal. Solid steel in the back at the bottom of the door hinges, but grinding it keeps coming up like foil. Mind you, the steel in the back does not grind shiny where it is rusted, seems to stay dark. Never really seen anything like this. Of course when you try to weld it, you just burn holes thru, so I am not even sure its steel.

Is this something common or did I just get lucky as usual?
 
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