What Carl52 said.
Use some scraps to set your amperage. They should be unsupported at the weld point like the patch would be. Crank up the amps till you burn thru, then back off just enough to get full penetration. Practice makes perfect.
Another trick is to avoid sharp corners on your patches. As large a radius at the corners will help reduce the warping.
It will take a looooong time to get that size of a patch done. If it takes days, that's what you need to do.
