#1) is using a flanging tool helpful for rookie welders to create satisfactory welds?
#2) I have heard that quenching tack welds and subsequent joining welds with lukewarm water will help prevent warping?
#3) Is some warpage expected when welding panels, even by expert bodymen and what are some things that can reduce the possibility of this?

if you are welding in a patch panel for rust repair you can use a flanging tool or lap weld the patch or you can butt weld the patch,,,i presume you are using a mig welder ,,if you butt weld you need to remember not to grind all the weld off so using a flanging tool would work good,,,when the patch is fit up and satisfactory to weld tack in each corner,,,cool the weld with a wet rag and make sure you dont leave the area dripping wet then finsh by making spot welds around the patch alternating from one side to the other cooling off with the wet rag as you go and remember to keep it dry and if you dont get in a hurry you should have a minimal of warpage,,,basically you will have a weld around the perimeter of the patch made up of spot welds,,,and remeber dont be impatient or get in a hurry,,,attention to detail

If you use the flanging technique to join metal panels, what should you use on the flanged area to prevent rust from forming between the the two pieces of metal that you joined before and after you weld?

use weld through primer. also put some por15 or similar product on the back if possible.

If you don't grind the weld down, how will the repair blend into the area? I saw on TNN Trucks! where Stacey welded a panel over a door handle pocket in a shaving procedure. He finished grinding and it was flat and flush as can be... You would never know there was a weld there!
What's up with that?
Mark

It's awfully hard NOT to get full penetration when welding sheet metal panels. There is no reason not to grind the bead down afterwards, unless you happen to be doing something like shaving driprails from a car and need the structural support. then you need to smoothe the weld enough so that a minimum of filler will make for a smooth finish.