Rear Fender Patch

I'm slowly working my way around the truck. I started with the drivers side front fender. I just patched the passenger side rear fender. Well almost I was just about to declare victory.... then I noticed some pin head size holes where I had been too aggressive grinding the weld joints.  When I attempted to fill in the holes, I blew right thru the fender. I almost cut it all out again, but I  kept trying by continuing to turn down the voltage on my  MIG until I found the sweet spot at 2.