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Shrink or stretch?

#1
I have a 57 Cadillac hood that has been through the ringer. It appears to have been buckled from either the hinges being frozen or possibly flying open. It was "repaired" with a pointed hammer and lots of filler. I have gotten the hood fairly straight, but can't seem to get that last bit of stiffness needed to keep it from oil canning. If I heat the area gently with a torch it stiffens temporarily, but when it cools, loses the stiffness. This hood scares me because of the almost flat crown in the rear. My question is do you think it needs to be stretched where i applied the heat with on dolly hammering, or shrunk where the "repairs" were done? I just don"t want a swimming pool!
 
#2
This will be difficult to answer without seeing pictures and really being able to assess your situation. More than likely if the heat corrects the problem the metal needs to be stretched as the heat is expanding it.

Generally the method to stretch that area and regain the crown would be to work the area "hammer on dolly". This displaces a small amout of metal and will raise the area.

I have used a slapper with a heel dolly on the underside to accomplish this without doing a lot of damage to a smooth surface with a low crown. If braces are in the way it becomes more complicated. Removing the braces on a 58 Caddy does not sound like fun but I replaced (and fabricated) the front brace on a 56 Buick this summer. It certainly is do-able.

John
 
#3
I just re-read your question and realized I missed part of it. You will need to decide which way to go. In theory, either stretching the low crown area or shrinking the stretched damaged area will accomplish the same thing but you have to decide wheather there is a low area needing to be raised or a high area that needs to be shrunk.

Hey, It's only metal! :)

Best of luck.

John
 
#4
I just re-read your question and realized I missed part of it. You will need to decide which way to go. In theory, either stretching the low crown area or shrinking the stretched damaged area will accomplish the same thing but you have to decide wheather there is a low area needing to be raised or a high area that needs to be shrunk.

Hey, It's only metal! :)

Best of luck.

John
I just re-read your question and realized I missed part of it. You will need to decide which way to go. In theory, either stretching the low crown area or shrinking the stretched damaged area will accomplish the same thing but you have to decide wheather there is a low area needing to be raised or a high area that needs to be shrunk.

Hey, It's only metal! :)

Best of luck.

John
Thanks for the reply! That is the debate I have been having with myself. I just thought that with the gentle heating showing an improvement, it must need to be stretched slightly in that area. The "repaired" part was a combination of lows and highs that I have gotten straight enough for a one coat of filler repair. One good thing about this hood is there is no bracing in the area of damage, so I have access. I will try to get pictures tomorrow.
 
#5
All of the "pointed hammer" repairs, coupled with your efforts to straighten them out, suggest that the area is stretched. Adding heat and getting a temporary repair suggests the need for stretch in that isolated area.

I would say to get a straight edge about three feet long or so, (ruler, flat bar of aluminum, etc.) lay it over at a slight angle so it better conforms to the hood's crown. Now look for highs, lows, etc. in multiple areas and directions and see if you can determine if it is indeed a large stretched area from all the pimple flattening. My fear is that you have a large area of stretch with one shallow area that doesn't quite have the same amount, so it APPEARS as a low, but actually is just surrounded by quite a bit of stretch.. I think if it does indeed have a large area of stretch, it will show up in the finished panel..
 
#6
All of the "pointed hammer" repairs, coupled with your efforts to straighten them out, suggest that the area is stretched. Adding heat and getting a temporary repair suggests the need for stretch in that isolated area.

I would say to get a straight edge about three feet long or so, (ruler, flat bar of aluminum, etc.) lay it over at a slight angle so it better conforms to the hood's crown. Now look for highs, lows, etc. in multiple areas and directions and see if you can determine if it is indeed a large stretched area from all the pimple flattening. My fear is that you have a large area of stretch with one shallow area that doesn't quite have the same amount, so it APPEARS as a low, but actually is just surrounded by quite a bit of stretch.. I think if it does indeed have a large area of stretch, it will show up in the finished panel..
Thanks MP&C! I think you are exactly right. I did have to shrink the "pointed" area which was WAY too high, and compared to what I started with I am happy it is this close. The shallow area really feels good, it just won't take enough pressure to sand or buff. I may still have a larger area that needs to be shrunk that I can't feel with my hand, and I will check it with a straight edge.
 
#7
A straight edge works great, but keep in mind that the panels are not flat, so you have to roll the straight edge slowly across the panel at different angles along the direction with the least crown. It doesn't work well on med-high crowns, and of course it has to be rigid. I have several straight edges, but the one that works best has a rounded edge.
 
#8
I didn't have a chance to work on the hood today. I couldn't really find a straight edge that worked like I wanted it to, (out of all the junk I have),so I will get some aluminum flat bar and try that. Took a couple pictures but need to get better ones with the straight edge. I just have to say I have doing paint and body work for over 35 years, but mostly on the finishing end, until the last 7. It really amazes me how much QUALITY information is on this forum! I have learned so much about things I was unsure of just reading the information here. Thanks!
 
#9
Here are some pictures of how the hood is as of now. The yellow "x" is where it wants to oil can. Other than that it is fairly straight, only very small highs and lows in the "repaired" area. I'm just afraid if I keep working that area it might lose it's stiffness there, too. caddy hood.JPG caddy hood.JPG caddy hood 2.JPG caddy hood 3.JPG caddy hood 4.JPG caddy hood 5.JPG caddy hood.JPG caddy hood 2.JPG caddy hood 3.JPG caddy hood 4.JPG caddy hood 5.JPG
 
#10
You have no choice but to work the high spots. Whether you use a hammer off dolly, shrinking disk or heat, the high spots must come down. I have never used a shrinking disc but I think that would be great place for one.

John
 
#11
I have never used a shrinking disc either, but this hood may give me a reason to try one. I am going to try a little off dolly hammering first and see how it acts. Thanks!
 

Chris_Hamilton

Trying to be the best me, I can be
#13
I would work it (planish, hammer on dolly provided you can get one behind it.) until I got a smooth low swell then get the torch out. Smallest welding tip you have, very low flame, start heating small areas, let it cool naturally. It may take a little trial and error when doing this. If you shrink one area too much. Stop and hammer on dolly that area until it returns. Or if you get more pronounced highs from planishing, heat it and lightly tap the swell (volcano or cone from the heat). You don't need to get it cherry, when you do this. Also you can continue to tap well after it starts to cool. Will continue to shrink just not as much. Don't quench it, heat small areas and lightly tap with a body hammer. No need to backup with the dolly just light taps in the heated area. Quenching of any sort should be considered a last resort.
 
#14
He said he can get behind it because there was no structure there, which has been my thought all along this thread. Did you remove the structure to get behind it, or is that the reason this dove and replacing a structure would fix this? I mean some 1/4 round bar bent to the shape of the outside would support this from the inside wouldn't it?
 
#15
I agree with Chris. On an area like that hood I would not use water to quench it. If you "cherry" up a spot the size of a dime or less it will shrink plenty. I work the torch in from about a ten inch circle, circling in to the bullseye allowing just a very small 1/2" area to turn red. You will see a large area swell up as you do this. Take your slapper or low crown hammer and tap the metel around the same area working your way in toward the area that is "cherried". That pushes the molecules into the soft heated center compressing them and when it cools and contracts, it will bring the panel down more than you will expect. I personally never use water which will temper the metal if you quench it quickly.

There will be those who will disagree with me but I have done it this way for 35 years.

John
 
#16
The only structure on this hood is on the front and rear. The center only has 2 crisscrossing 1/4" bars. I have been working it with heat and off dolly hammering and seem to be making some progress. I have been going slow and gentle, trying to sneak up on it. Thank you guys for all the advice.
 

Chris_Hamilton

Trying to be the best me, I can be
#17
I would add to what I posted that by dollying the area out until it is swelled allows you to have a pretty good idea where to shrink it. Highest point of the swell will be the most stretched, that would be the area that needs the most shrinking. Work out from there. Helps take some of the guesswork out of where to shrink.
 
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