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Old 02-13-2004, 06:52 PM   #1
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MARTINSR"S "Flat panel basics"

First thing is whether or not it really needs shrinking. It may be a panel with some crowns in it or something like that. You could have a panel with a "soft spot" in it simply because the panel has lost it's shape. Below is the "Basics of Basics" to flat panels.

Flat panel basics.

When you have a large flat panel that is flexing the first thing you need to do is find out why. Sometimes you can stop it, other times you can't. But if you can stop it, you'll have a much easier time with the body filler work. Hoods, decklids, and the roof are particularly difficult because the heat and weight of the plastic filler can have an effect on the metal. The good news is many times it is very easy to repair.

First off, there is no such thing as a "flat" panel. All panels that appear flat actually have a slight crown or gentle bow up in the middle. Go to a flat panel and lay a straight edge across it. You will see that the straight edge in not touching the panel at the on the outer ends. If the panel were perfectly flat it would appear to the eye to be concave. It would also have no "body" and flex very easily. This is the problem with your large flexing panel; it has "lost" its crown and is now weak and flexible.

The first place to start your search for a culprit is under the panel. Many panels have inner structure that supports the outer skin. When the outer panel has been damaged the inner structure was bent down along with the outer. This inner structure can be in the form of just a simple inch or so wide support running across the panel to the complete support by a stamped panel that goes covers the underside of the panel. These full inner structures can commonly be found on hoods and decklids. The inner structure can sometimes be bent down, causing your flexing. It usually is very close to the outer skin, with just a thin layer of a foam or urethane adhesive. It may have small "dollops" of this foam or adhesive that has been squished between the inner structure and outer skin or even a thin piece of tarpaper.

You can push up on these low spots to return it to supporting the outer skin, as it should. But it is difficult because you can't push it past where it needs to be. On this particular type of damage, the inner structure would need to go past the correct shape and then "relax" back down to where it belongs. It can't do this of course because the outer panel is there and limits the inner structure from going up where it needs to go. Just as with looking at the "big picture" when you look at any dent, you need to search for a kink or bend that is holding the inner structure down in that area. If you apply pressure up on the low area and tap out these kinks, you may get it to stay back in shape. If these methods don't get it back up to support the outer panel properly, you will need to "shim" between the two panels to get the outer panel up where it belongs. This can be done with sheet of tarpaper or more adhesive. As a last resort a thin piece of wood like a paint stirring stick can be used. Of course, this is a little on the funky side but if you are haven't been able to correct the problem, something has to be done. What you have to watch out for is applying too much pressure in one area. If you were to force a piece of wood in there, you will likely be making a high spot on the outside. That would just give you in a whole new problem.

Sight down the body lines that are nearest the low, "oil canning" , or just plain flexing area. A body line is effectively the "edge" of the panel. Those crowns in the flat panel that I mention end at the body line. So each area in between the lines is sort of like an individual panel. Look to see if the body line is low, it may be holding down your panel. If it is, you need to push it up. To help you determine how straight the line is sometimes you can use a metal ruler as a "straight edge". How can this be done on a crowned panel you ask? A metal carpenters yard stick will bend very easily, right? So what you do is lay the yard stick on it's back against the panel and apply a little pressure on the outward edges low area where the metal is OK. You will then have a "curved straight edge". I have a drawer with a number of these metal or aluminum rulers in it and find them very useful. I treat them like rice paper and they will last a lifetime.

So lets say that you have found that you have no low spots in the body lines or there were one or two and you repaired them. Now you have to look for something else that is holding the panel down. This can usually be found in the form of a "crown" or "brow". When you put a dent in any panel the metal has to "go" somewhere. All panels have this crown, right? So as an example picture a metal rod that is 3 feet long. This rod has a slight bend to it. The center of the rod is up from the ends about three inches. If you were to push down on the center, the rod would get "longer", right? So, if the ends of the rod were clamped in vices, the "extra" rod would force the areas on the sides of where you were pushing to go up. You panel does the same thing only on a much smaller scale. Most brows will be found on the outer edges of a panel, this includes of course at the edge of the body line. They are VERY common around the outer edges of a roof. Search around the outer edges of ANY bent roof and you will find them.

The brow or crown is a U, C, L or even I shaped high spot. In the center of that curved high spot is a low spot, sort of like a "pocket" in the brow. Just one or two of these will make a panel, especially a large panel look like a cotton sheet! What you have to do is to push up on that low spot while tapping down on the brow. When I say "tap" I mean TAP. Just the weight of the hammer bouncing off the brow will do it sometimes. Use a large VERY flat body hammer or a flat body spoon for this repair. If you are careful you can repair these brows with little to no plastic filler. Just take you time and keep checking the area with a block with sand paper or a vexon file if you have one for low and high spots.

Now, if you simply can eliminate the brow and low spot, you have won the battle. If it takes some plastic filler, so be it, you have given the panel it's strength back and that is what matters.

If you do need to shrink, below is the "Basics of Basics" to shrinking. And yes that shrinkinig disc does work, I have one. It isn't magic though. An understanding of shrinking and metal manipulation is still needed.

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!


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".

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