This is a picture of a part of an old trunk pan. I cut a piece of metal out of it because it's kind of worn and pock-marked, like the kind of metal you would blow through when you're welding. These pictures weren't real clear so I went back out to the garage and got a couple more pieces of metal and took some more pics.
My intention is to show you how to put a piece of metal in an area that's enclosed like the back of a trunk, so you can get some metal for backing, and you can concentrate your weld on the new metal and let it flow into the older thinner metal and save the blowouts.
Here's the cut-off wheel I used. It's about 5/64" thick or .080. Behind that, is a dremel with a reinforced dremel cut off wheel.
You can see the slot I cut with the regular cut-off wheel - .080 then I took the dremel and put it in at an angle. That gets the burr off the back side.
This is the backing plate I cut. It's about an inch wide and an inch longer than the slot I cut. I drilled two holes in the backing plate about 1-1/2" apart with a 3/32" drill. Then I took about 8 inches of .023 mig welding wire, ran it through the holes and made a loop, then folded the wire up tight to the backing plate with the two ends sticking out and pushed it down through the slot. That way you can hold on to the single strands of wire and the plate will flop inside the hole then you can pull it up tight. Just pull on the 2 pieces of wire and it will hold the backing plate tight to the piece you're welding. In the picture you're looking would be the back side of what you want to weld.
Here I'm pulling up the backing plate tight to the other metal. I just hit it with the welder in the center. Now you can concentrate your weld on the new metal in the slot, and let the weld flow into the thinner metal. This should keep you from blowing holes in it.