In answer to your question about heating the high spot or low spot, the answer is yes. Remember the high spot on one side of a panel is also the low spot on the other side. It may help you visualize everything if you outline the area of the oil canning. I'd recommend using a piece of soapstone or chalk, or a paint pen. If you use a Sharpie just make sure it is 100% cleaned off or it can bleed through top coats. To find that area start lightly pushing on the panel and make it pop in and out (oil canning). Move outward until the metal feels stiff again and won't move under light pressure. When you find that spot, mark it. Then start working around in a circular area attempting to locate the stiff areas and flexible areas, marking the "divide". Once you're all the way around the area you should have a somewhat circular or oval area. Find the center point of that area and that is where your maximum stretch is (or, the "high" point). This is where you'll start applying your heat initially. Depending on the size of your circle (oval, whatever shape), I wouldn't start heating an area larger than a silver dollar, smaller still if your stretched area isn't that larger. Heat and quench. Check to see how much it tightened the panel. It WILL be noticeable, even with the first time. It probably won't eliminate it, but you will notice some difference. Start working small areas around your starting point working your next spots in a circle around that point. Heat, quench, check. Through the shrinking you're drawing that "high" point back down and, basically, tightening it back up. You should not have to work all the way to your outer lines. If you find you're getting out there, move back in and start working back out again. While it sounds time consuming, it's actually a pretty quick process.
If you have dents and creases still in the metal in the area you need to shrink you can hammer and dolly them prior to quenching. Just don't touch the metal with bare hands! If you mess up and touch it, trust me, you'll only do it once!
Also, if you hammer and dolly remember you're not whacking away directly on top of the dent/crease to flatten it out. Place the dolly on the back side and gently push out. Work the hammer around the perimeter of the dent/crease using light taps and almost a pushing motion as the hammer strikes the sheet metal, pushing towards the center of the dent/crease. The metal will be soft due to heat so use very gentle blows.
If you want to make an "oil can" dent I don't recommend using a piece of scrap metal. If you can find a destroyed fender or door it'll be easier to replicate. Locate the area with the least support (i.e. the broad area of a fender away from formed body lines, bends, creases). Use a large nylon or rubber mallet and make a nice big dent. Try not to make a crease in the dent. The metal has now stretched away from the blow. Now take a body hammer on the back side of the dent, supported by a dolly, and start flattening the metal from the outside edge of the dent inwards (towards the center). Use good strong blows so you really hear a good click on the dolly. After working it flat it should be nice and stretched because of how you worked it. Now, practice away!!
I didn't intend to make this reply as long as it has gotten, but discovered there's a lot in explaining as opposed to showing! Sorry for that. Thanks very much for the compliments and again, good luck.
