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” think 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!

Precautions:

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, the
dolly 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”.

MARTINSR, great info. But I have to ask, are your fingers tired?.....J/K:)

Another effective way to shrink that I use often (When you can get behind the work) is to use a shirnking hammer and a shot bag. I make my own shrinking hammers and put sharp serated points on the face, sort of like a framing hammer. I use the hammer on dolly method when doing this.
The serations "grab" the metal and pulls it together. The shot bag absorbs the blows and does not stretch the metal like normal hammer on dolly does.
Try this experiment. Take two identical pieces of paper. Wad one up real tight in a ball. Then gently "unwad" it as flat as possible. Then lay it on top of the other piece. You'll find it is smaller than the unwadded piece.
The shrinking hammer uses this principle to shrink.

Great info I tried heat shrinking my door and it worked great.

Cool Info!
I see that you are in SF. Do you have the capabilities to show me each method you described in your post. I would be willing to come out to your place for a few lessons, and pay you for your time in cash, food, beer or whatever. Let me know if that interests you.

Martin, How hot do you heat it? Cherry red? If I have a spot that I have worked too much (thinned the metal) and I am trying to lower it, do I heat it red hot, hit it with slapping spoon and quench with air? What happens if don't quench it? Will just swell back up when it cools?

Thanks

Originally posted by MARTINSR
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.
How do you determine the metal has to be shrunk? Thanks

John, I barely have time to spend with my kids, wife,and caring for my elderly parents! No, I am sorry but I really just don't have the extra time.


stllookn, yes, as hot as barely red. But as I mentioned there are other tools that don't get it anywhere near that hot. So it is a judgement call. "If I have a spot that I have worked too much (thinned the metal) and I am trying to lower it, do I heat it red hot, hit it with slapping spoon and quench with air?" all or none of the above. :) It really depends on the spot. But if you were to heat an area and with a dolly behind it in an OFF dolly fashion tap the heated area down, that will "gather up" the metal in that area, shrinking it. This could be done without any dolly too, it depends on the support of the surrounding metal. If you were to heat a little spot in a firm area like near a body line you could heat, then quickly tap it down lightly with your flat body hammer. Or you could not hit it and cool it down to "gather" up the metal.

68 Suburban, that is a tough one, it is not easy some times. It is usually needed because metal is stretched.This could be found in a high spot in the metal or a low spot that doesn't want to come out. It could be in a high or low spot that when struck goes the opposite way. First it is low, you hit it up and now it is high. First you have to follow all the suggestions in my "Basics" on flat panel repair. These tips apply to all panels flat or not. There may be an area that is "holding in" the dented area. Or an area that is taking the rigidity out of the panel.

Martin, Thanks for the help. I am going to practice on a few spots. I think I understand it now.