Where is the dent and how big is it? These may be deciding factors in how you repair.
If your panel needs is shrinking. You can do this in many different ways, again depending on where and what size the dent is.
You could use:
A torch. This can provide the most heat for shrinking, consequently the most DAMAGE.
A shrinker attachment on a spot or MIG welder. This is a great way to shrink small dents or thin metals.
A grinder. Yes, with an 80 or 100 grit disc and a lot of speed you can heat the high spot and cool it to shrink, without taking "much" metal off.
A "DA". With it in the "grinder" mode and some 120 grit, use it the same as the grinder. I do this all the time, it is very useful.
A Heat gun. There are electric hand held heat guns that provide you with 1000 to 1500 degrees of heat.
Simply use an “off dolly” technique with a hammer and dolly. Push up on the panel with the dolly, then tap around the area OFF the dolly. Many times there will be high spots anyway around a low spot so this works perfect. Remember, do not hit the hammer where the dolly is. That is called “on dolly” and STRETCHES the metal.
There are different methods for shrinking, you heat and cool or you heat and work.
Heat and cool is usually for areas that you can't get behind, very small low spots, or very thin metal. This is the easiest way in that you just apply heat and then cool with air or water and the area will be shrunk. Cooling with air really works well.
I have done some little tests and blowing the heated area with a blower on your air hose shrinks almost as much as quenching it with water, without the rusting concerns. When the metal cools the molecules get closer together in the heated area, thus pulling in on the surrounding area
and shrinking the surface area of the panel.
Heat and work is a bit trickier. You heat (usually with the torch) and put a dolly behind the heated metal and gently strike the heated area with a hammer “On Dolly” (the largest, flattest hammer you have). When the metal is hot the molecules are free to move about. So after heating and before the metal cools, tapping on the heated area (that has either raised
or dropped) allows you to “push” them to where you want them.
You have to do this carefully, because if you hit “On Dolly” too hard, you will push those molecules apart, and make matters worse! The idea is to gently push the molecules to the center of the heated area and this will “pull” in on the surrounding metal. Picture a 12 x 12 inch 1/4” thick tile of playdough. If you maintain the 12 x 12 but thin the Playdough
down to 1/8” you would have a big hump in the middle right? Well this is what the metal is doing, you have to move the molecules like the particles of playdough back to the proper place. When you heat that sheet metal, believe me you don’t have to hit it much harder than if you were hitting Playdough, so be CAREFUL!
Bare metal is the best to shrink, especially the back side. BE CAREFUL THAT THERE IS NO UNDERSEAL TO BURN, and besides if you are going to “work” it cool, thedolly gets all covered with underseal.
Get a partner to hold the torch while you “work” the metal. And this partner can also keep a fire watch.
Make sure you know what is behind the metal you are about to heat, wires, lines (like FUEL!) should be removed. And that sound deadening material can be VERY flammable (I know all to well about that one!) .
The metal will only do what you tell it to do. It has a memory and you have to “help it to remember”.