Handtool method

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Look at the back of the chair and determine where the spot welds are that hold the back on. Turn the chair over and hammer the chisel in at the welds to break them.

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Continue to work your way around the chair breaking welds and prying the chair back off.

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File down the rough spots and welds then use a piece of emery cloth to smooth to the touch.

Run your hands over the finished chair to check for anything sharp that might cut. If left unpainted the chair is unlikely to rust severely in normal indoor use. If you want to be sure then paint or clear coat the bare metal.