Dude,I could not have said it better and I have lots of body shop experience!
I will add one thing. Very helpfull in controlling heat while welding,compressed air can be used to cool things down.
O.K ,one more thing..lol
Take your time while welding. Stitch weld around the perimeter. Dont start with a corner or edge. Tack it down in the middle of a seam. Make sure you have some gap iin fitting the panel. This will allow for expansion as the metal heats and help prevent warping. I butt weld and make an even gap by running my small zip cutter through each seam. Dont remove tons. Just ensure the gap. Hardly any grinding is needed with this method.
Good luck and be patient!Body work takes time. If you get frustrated/impatient take a break/have a beer.:)
HASTE MAKES WASTE!