Hey Guys,
I have been repairing the inner fender on my 67 Mustang coupe that was a little mashed from a slight front ender. Hammered and dollied the top pretty flat, but had some oil canning. Read up on heat shrinking and heated up the area using a Bernz-o-matic propane torch (I do not have a shrinking disc) and quenched with water. Definitely tightened up the metal. However, the high spot gets even higher when heated. I know metal expands, but I am having trouble getting it totally flat. After heating it, I put the dolly on the high spot and tap around it from the opposite side (off dolly?). I can get it pretty flat, but then the metal stretches again and I get some oil canning. So when I heat it up again to shrink it just raises up high again.

I am new at this, so obviously I am doing something wrong. Any tips, advice would be greatly appreciated. Overall I have it pretty much back into shape, just looking to get that las 10-20% nice and flat.
Thanks in advance!

Metal shrinking is tough and takes a bit to learn.

The propane torch is not really the right tool to do heating.

You need to get the spot red-ish hot (and fast) only where you want to shrink it.

Hammer and dolly as you were working outter to inner around the redish cherry. Then quickly quench with water.

If you are having difficulty with "high spots getting higher" one problem is the propane torch not getting it hot enough in the right spot; the other thing is you can try is heating it from the reverse side if you can.

 

The quickest way to ruin a panel is to heat shrink it when you do not understand metal and the damage in it. I cringe when I hear people talking about needing to shrink.

I know, I have done it, I have messed up panels. Now I know better.

A lot of oil cans are actually from shrinkage. For example, take a piece of paper and crumple it up. Now flatten it out and put it over a flat piece of paper. Notice it has shrunk.

Now you have shrunk it some more and if you have shrunk into a high crown area you have caused difficult to understand and fix damage. It is easy to do, I have done it. After reading some books and watching some videos I have a basic understanding and can keep out of too much trouble.

Now to answer your question, you have troubles. You have worked the metal and done some shrinking. It is out of wack and only someone who understands metal and looks at it might be able to give you advice that works. You need to figure out how to stop the oil can and limit further damage and just build the area with plastic.

You should look for a book on autobody repair. A textbook from a votech course from around 1970 is what I found to be most helpful. I stumbled onto it and found it had a good section where it talked about metal and how it moves. After reading it and playing with what I learned 3 or 4 times I started doing better body work. You might be able to find a good book at your local library. The HP books and other books on metalworking are not too helpful for basic learning. They do not talk enough about how and why on moving metal and then they start showing how to do a job with some very expensive tools.

I also like to point out that EVERY weld is a shrink point. Keep that in mind as you put a panel in. Also keep in mind shrink damage may not appear near the shrink point, sometime the damage is across the panel. It is a very complex balance.

 

Thanks for your reply. I have tried both heating it from the high side and the concave side. same result. High side gets higher. I have also used the dolly on? technique, working around the high spot in towards the high spot. Again, I can get it almost flat, but when I go to shrink it again, the whole thing just raises up again. Vicious cycle. :spank: Its not the end of the world, it is only the inner fender which I can slap a little bondo on, but I would like to learn how to do it correctly for the future and other body work in mor noticable areas. I like the idea of being able to repair a dent with little or no bondo. If nothing else, I am persistent. Just need someone to show me the error in my ways.

Another thought, the propane tanks come in a high temp version. I think the berns-o matic is a yellow can. would that work better?

Is the idea to heat it up faster?
Should I heat and quench first? or heat, hammer, dolly, quench? or vary the method.

I have also read that it is not req'd to get the metal cherry, but only til it blues and sizzles when quenched. Is that accurate?

I think this is a lost art, as most people just use a slap hammer and slap on the bondo. But I would really like to learn how to do this properly. It seems like the best method or repair. I am suprised more people don't want to learn about it.

Thanks again.

 

With a flat piece of sheet metal the bulge will always be towards the heat. When you first put the flame to the metal that side will be slightly hotter than the back side causing the flame side to expand first.

Any time you heat metal up to where it turns blue you have just cause a shrink point. The metal shrinks because it has gotten to a plastic state and the the surrounding metal is pushing into the plastic metal. When you take the heat away the metal cools too fast past the plastic point and holds the shrink. Notice you do not need water to hold the shrink. Shrinking should be done with a hotter torch if you want some more control over the shrink area. The general rule is to keep the red spot less then a dime size.

Hammer on dolly hits will increase the shrink, if you know and understand how and when to do it.

You always let the metal cool to a uniform tempurature before deciding if you need more shrinkage. The warm metal in the shrink area will still feel high, patience it needed when shrinking.

Now you must understand, most of your repairs will acutally be stretching. Most people have a mistaken impression that an oil can means stretched metal. All an oil can indicates is a differential in the size of an area of a piece of metal. Most damage is because of shrunken area that needs to be stretched. Usually you have some damage in a curved area that causes a pulling of the flat area that causes an oil can.

How damage and welding causes shrinkage is covered in depth in the autobody text books.

Here is some more stuff I have written up on metalwork:

http://home.comcast.net/~68c/Metalworking.htm

Keep in mind metalworking takes hands on experience to learn after finding a source of factual information. It also takes a nice assortment of tools. Some you can make (a slapper from a leaf spring) and some you need to buy good quality (your main hammers and dollies, you should have at least two quality hammers and dollies). The taiwan hammers and dollies will mushrum out if they are your main hammers.

BTW, I started with a 66 mustang fastback, did a lot of learning the hard way on it. Now I can put in panels made from scratch that almost do not need bondo with a TIG welder.

 

First off, Rambo, you do NOT need (or should you even want to) get the metal red hot to shrink. It only needs to be heated, red is WAY too hot.

Coda, your first line in the very first post tells me you are going in the wrong direction and must stop now.
"my 67 Mustang coupe that was a little mashed from a slight front ender". That metal isn't "stretched" at all guy, the panel has been "shortened" by the impact of the collision.

Lay a piece of paper on the table in front of you. Now holding the left side of the paper flat on the table with a few fingers, slide the right side that is loose on the table toward the left side.....the paper "humps up" in the middle right? The paper doesn't need to be "shunk" in the middle, it needs to be returned to the original "length" to be flat once again.

Your inner fender is "shorter" and needs to be returned to it's original state FIRST before you do anything. We are only talking about a fraction of an inch. So the upper measurement of your structure may look fine. But I will bet you there is a eighth inch or so there that you could find to push the rad support forward and correct this.

Brian

 

Martin Sr, Thanks you sir.
Maybe I should clarify my problem and what I am trying to fix. The car, before I got it, was in a minor accident. i do not know what it hit, but it hit something in the front left side. The bumper was pushed up alittle and the headlight bucket/fender extension mashed a little and left front corner of hood bent a little. This in turn pushed the left fender up and back a little and the rt fend out a little. the pulling of the fenders cause the top of the inner fenders to buckle a little and pull the divets where the bolt clips sit either flat or raised and rinkle the inner fender tops.

So, as I had read in the forums, it is best (?) to try and work out the damage in reverse from the way it was caused. So that lead me to the rt rear inner fender behind the shock tower. Using a hammer and dolly, i reshaped as best I could and also had to reform the indentation where the fender mounting clips reside. I did a pretty good job reforming. Looks almost like to original undamaged ones.

However, after hammering this small portion of the upper fender, and reshaping the indent, I was getting some oil canning. I was under the assumption that when you are hammering, you are mashing the metal thinner and thus stretching it, causing the oil canning. Also, being the the recessed portion was pulled up by the fender, I deduced that it was stretched as well. This brought be to metal shrinking, because as I reshaped this recessed portion, the flat portion seemed to bulge. the rest of the story is recapped above. In fact, when I did heat and quench, the oil canning was cured and the metal stiffened up, but then after hammering down the high spot I would get some oil canning again, heat again etc.

I appreciate all of your input. More information is greatly appreciated. I figure, this portion of the car, which will be mostly covered by the fender is the best place to learn before I move on to other panels. Please point out the error in my ways, assumptions etc. I love constructive criticism, :boxing: as it only helps me improve my skills.

 

Lost in NJ, Very good information :thumbup:
Bob

 

From my experience when using heat on a panel,the type of hammer used is of utmost importance.A normal panel hammer,usually a flat round face ,is no good for shrinking.What you need is a proper shrinking hammer,with what looks like a knurled face.
This has the effect of dragging the metal inwards when done in the right fashion ie heat the hight spot and hammer on dolly from outside towards centre from all directions.
You really need to see someone do it

Whenever you hit metal with a flat hammer you are actually stretching it,remember that

 

Unlike Lost in NJ, I have found that most, but not all oilcans seem to happen because of over stretched metal....may be differences in the type of work we do. It does not matter which side of the metal you heat from. Shrinking hammers are crap...no offense meant! Hammering to shrink is better done cold than hot by a beginner but still only in rare instances. You can easily cause more stretching or distortion hammering on hot metal. Friction (heat) shrinking alone is much more controllable than heat then hammer shrinking. For a beginner, I believe it is easier to raise/stretch all the low spots, smooth with lots of on dolly work and then shrink once you have a smooth but slightly high surface. Do not worry about every hammer blow stretching the metal or you will not get the metal smooth. Opinionated aren't I?..(insert smily emoticon)

John

 

We all have opinions , If the length is right and the bulge is there then the metal is definately stretched. All of the oil canned areas I've ever repaired were stretched. When a panel gets hit hard enough to cause a bulge there's usually enough tension on it to not have an oilcanned condition(stretched). The proper way to repair that apron would have been to put pulling tension forward (direction opposite of that which caused the damage) then work the wrinkles out with the pressure on. A unispotter works well for the average person and is fast. If an oxy/acetelyne torch isn't available you could drill a series of 1/8" holes and mig weld them up quenching each as you go. Severely stretched areas can be reduced by slicing a + in the bulge then hammering to shape and welding the slice-or just section it out and replace if it's really thin. I've seen some older "metal finished" repairs that were so thin from picking and filing that there was no saving them and sections needed to be replaced.

 

First off, Rambo, you do NOT need (or should you even want to) get the metal red hot to shrink. It only needs to be heated, red is WAY too hot.

Coda, your first line in the very first post tells me you are going in the wrong direction and must stop now.
"my 67 Mustang coupe that was a little mashed from a slight front ender". That metal isn't "stretched" at all guy, the panel has been "shortened" by the impact of the collision.

Lay a piece of paper on the table in front of you. Now holding the left side of the paper flat on the table with a few fingers, slide the right side that is loose on the table toward the left side.....the paper "humps up" in the middle right? The paper doesn't need to be "shunk" in the middle, it needs to be returned to the original "length" to be flat once again.

Your inner fender is "shorter" and needs to be returned to it's original state FIRST before you do anything. We are only talking about a fraction of an inch. So the upper measurement of your structure may look fine. But I will bet you there is a eighth inch or so there that you could find to push the rad support forward and correct this.

Brian

In my defense I did say "red-ish" - that's not "cherry red" where I come from - but I could have clarified more...