Use the grinder to sand down part of the metal fence so the ground clamp on the welding machine has ground to be on for the machine to work.

What the ground clamp does is that it provides you with the right voltage that you will need for the metals to actually be welded together. If it isn't a similar metal, the voltage/power of the machine can either be too little or too much.

Put the welding ground clamp on the same piece of metal that has also gotten paint removed with the grinder.