How far do I need to cut to get to good metal to weld?

I did some thin material welding with my old old mig (thats 2 migs ago) that only had switches for setting heat.
Learning the limits of the metal is key. Start low and work your way up. Wire feed will make up for some issues but not all.

Copper block for backing is your best bet,it draws heat the quickest.
Compressed air will aid in cooling the HAZ (heat affected zone) and will aid in the event of a flare up.
I usually blow the fires out,save the water if it gets out of hand.
Oh,a pail of water and a rag Should be handy when welding anything.

Practice before starting on your actual panel, finding a comfortable work position makes a huge difference in your welds too.