View Full Version : Metal Roof Flexing/Oil Canning..Shrinking Questions...UPDATE
12-28-2004, 08:57 PM
Well I am new here so here we go.
I've been restoring a 69 Dodge Charger for quite some time. Doing bodywork and everything else that goes along with it. A lot of sheet metal replacement.
Now I'm pretty much on the last portion of the major body work, the roof.
I had two portions in between the roof braces that looked almost perfect but when you applied any pressure they would flex or oil can.
The first portion on the front I was able to shrink with the shrinking tip on my stud welder. I know have a slight low spot, but a skim coat of filler should take care of it.
Now as for the back portion, I'm having trouble, and maybe I'm making things worse.
I attempted to use the tip on my stud welder to shrink it as on the front, but no luck there, so I ended up making a small shrinking disc out of a stainless bowl that will fit on my 4.5" angle grinder. I'm attempting to use it. I did get some of the area shrunk and it looks nice, but there is a strip right in the middle of the area, the area I'm working on is about 1.5 feet by 2.5 feet, that is a high spot.
The problem is when I apply any pressure at all, whether from the disc, or anything it immediately oil cans most of the area causing a depresion of about 1/4" Way to much. I don't have any idea how to take care of this. If I try to use the disc it just oil cans down and that won't work to shrink it I don't believe.
Any maybe I'm not even doing it right. I'm useing the edge of the disc, and going over a small area, and then quenching it with a rag. It seemed to work well on the majority of the section in question, except that strip. I can post some pictures if that will help but I don't think they will show much.
Thanks to anyone for any help
PS I'm located in central Iowa, any members around here? Thanks, Adam
12-28-2004, 10:48 PM
Adam, welcome to MetalMeet. There are tons of forums on shrinking.....you can get the education of a lifetime here if you just dig a little.
Without a picture, and it being in the middle of the top, I hesitate to recommend anything but a shrinking disc. I don't think you are reaching the necessary heat range with the 4.5 in grinder and the small disc. You really need a larger high speed grinder (Harbor Freight sells a cheap one) and a disc of about 9".
You said you were using the edge....the object is to get the portion of the top you want to shrink almost blue then quench it. Use the flat of the disc to achieve this. When working with small raised areas, the flat also allows you to only heat the parts above level. If you can't get down in the oil can, go in after it.
I am assuming you are at bare metal. If you start to see galling, stop and smooth the disc with sandpaper til smooth, then use a little soap as a lubricant.
I just got a new disc today from Wray Schelin http://www.metalmeet.com/forum/viewtopic.php?t=2157 and man it looks nice...plan on using it tomorrow!
Good luck, and there are guys here with a lot more experience with a shrinking disc that may be able to help you more....john
12-29-2004, 07:09 AM
Maybe I better see about purschasing one. I may try just working on that spot with mine.
Yes the roof is down to bare metal. I guess I have not heated it enough. It has not turned blue at all. Of course I was afraid of doing it when it oil canned figureing it may very well do the opposite and strech it, but I guess that is not the case.
I will snap a picture sometime today and post it so you can see what I am looking at.
Thanks again for the help. Adam
12-29-2004, 10:23 PM
Was the car hit in the rear or side OR someone laid on the top?
Don't work the center of the oil-can. That will give you another low spot. Oil canning is usually the result of too much metal in a surrounding area. The convergent will be the "loose spot". Try working (shrinking) the area around the oil can.
A dent on the side of the top can cause the crown of the dent to push the metal towards the center and cause oil-canning in the center. It can also draw metal from the sides and "squeeze" the center causing oil canning. Find the cause and work that area rather than the result.
If the car was ever hit in the rear or side (quarters), the result can be metal pushed or pulled at the sail pannels. This disrupts the top metal too. Stand back and take a close look at what really may be causing the problem.
Hope this helps.
12-29-2004, 11:50 PM
Ken hit the nail on the head. http://www.metalmeet.com/photopost/data/3160/4hammer-vi-thumb.gif
12-30-2004, 07:15 AM
You've already received good information...one more thing....you might prop up the floppy part temporarily while using a shrinking disc...or shrink from below with a low crowned disc...this way you are pushing up at the same time you shrink.
Also look for small low areas away from the oil can that may need to be stretched up to put some tension in the panel, then possibly shrunk down a little after you have taken care of the oil-can.
And one good shrinking disc link deserves another:
12-30-2004, 07:28 AM
Wow great advice. As far as I know it was not hit in the rear, at least not hard. I have only had the vehicle for about 8 years and it's been off the streets that long.
I'll look around on the roof and see if I can see any low spots or any spots pushing the metal in.
Maybe I'll also try the shrinking for underneath as well.
John, I may very well be ordering your disc very soon I hope. Thanks for the help everyone, I'll try to keep you updated, I may have some more questions. Thanks, Adam
12-30-2004, 10:54 PM
I had a total of 3 440 Magnum Charger R/Ts (2 1968, 1 1969) back in college.
The first one was a 68 that I really liked. Unfortunately, I was tooling down a country highway in Nebraska one day at 70+ and had a large farm truck run a stop sign on a cross-road right in front of me. I left some brief skid marks and t-boned him right in the rear duals (which kept me from going under the truck).
Fortunately, the truck was empty (on its way to pick up a load of wheat). The force of the impact spun this big truck 180 degrees and put it off the road backwards in the ditch. If it had been full, it probably would have stayed where it was and I'd probably be dead.
Fortunately, all three occupants in my car were wearing 3-pt harnesses and weren't seriously injured.
My car just accordianed forward - everything kept moving. It was a LOT shorter. Every body panel on the car was significantly distorted except the trunk lid and it popped open. I think all the glass cracked or broke also.
The highest salvage bid the insurance company could get was $129.00 so I bought it back from them.
I then proceeded to buy R/T numbers 2 & 3. Number 2 was a good solid car.
Number 3 had been slid sideways up a guy wire on a power pole and had severe rocker panel and passenger door damage (although the door still opened and closed!) but at the time it seemed too cheap to pass up. At that point, I had no capability or knowledge to attempt to deal with the damage myself and it was severe enough not to be a commercially viable repair. It was a good driver, however, and I let my younger brother drive it for a year while he was going to school.
I really enjoyed my trio of Chargers. Big, crude, noisy and fast!
Good luck with your project.
12-31-2004, 07:07 PM
When I got back from RVN, I traded a buddy my `65 plymouth Belvedere (383 mag from 69 R.Runner & 4-speed)for a `70 Charger R/T, and all the parts to fix it. It was black on black with black . Spent my leave rebuilding and painting it, finished it the day before I had to leave for Ft Bragg......it had 440 mag and Torqueflite.......one bad mofo.......first wife wrecked it. Ran her off and traded for a `70 Challenger R/T SE 440 TF with the works........jb
01-09-2005, 11:20 AM
Sorry for not getting back to this post. I was getting some things finished up that I knew I could get done before I had to leave.
I took some pictures before I left so maybe you guys can give me some hints as to what tools I need to get and how to do the job, I really have no idea what I am doing.
We'll start with the front section, to start with it flexed quite a bit and oil canned. I shrunk it, but now I have a low spot, it doesn't flex anymore though. Did I mess it up beyone repair?
Here are a few pictures.
The rear section is the section I can not get to do anything. As suggested I looked for damge in teh immediate area of the 1.5 foot by 2.5 foot area that is the problem and I can not see anything, everything around it looks good. What I think I have is two low spots on the outside of the area, and in the middle I have a high spot, and when any pressure is put on this spot it oil cans. What can I do to fix this. Here are some pictures of it.
If a shrinking disc will do it that is what I will get, but if there is something else I need I will get that as well. I won't be able to get back to it for ahwhile so I have time to think about it.
Peter do you still have any of the three left. That's quite a story about hitting the truck. Sounds like you turned out to be lucky. Thanks for the good wishes on the project.
Sounds like you had quite a few sweet mopars. To bad teh Charger was wrecked. Do you still have the Challenger? Sure sounds like a nice one.
Thanks everyone, Adam
01-09-2005, 11:59 AM
I wish I did Adam............john
01-09-2005, 12:30 PM
Hi Adam Mopar
It looks like you need to stetch the areas that are too low. If you think about it this way..... you have a set distance between two points and a line (the metal) between those points,and you want the line to curve up without changing the distance between the points, you need to lengthen or stretch the line.
Low crown panels like this are tricky go slow and keep checking your progress.
01-09-2005, 12:49 PM
Thanks for the help Steve, how would you suggest streching them? I don't really know how to go about it. Thanks, Adam
Sorry to hear that John. Thanks, Adam
01-09-2005, 01:00 PM
Peter do you still have any of the three left.
No, I got rid of the last one in 1972 or 1973. I traded it in on a 1972 Toyota Celica, my first foreign car.
01-09-2005, 06:39 PM
Adam , As everyone was saying usually an oilcan in the center of the roof like that was caused by a buckle on the round edge of the roof close to the outer edge.With all of the surface ground there is no way to see the evidence of the original damage. Thes creases on the dge of the roof were usully made by something dropping on the roof and the place it would crease was on the curved edge of the roof.If these were not properly repaired you would end up with wrinkles in the center of the roof.
The area that you are showing is over shrunk like it may be you have to have very long arms and hold a low crown body dolly underneath the low spot and hammer on dolly moving the dolly and hammer marks evenly over the depressed area and this will slowly restretch the area that might have gotton over shrunk. When you hammer on dolly you will 1/4 inch round contact area from the hit and you do a nice pattern over the low spot. You should be able to check it after each series of hits and see it come up a little at a time. Dutch
01-09-2005, 07:26 PM
Thanks for the tips. I don't really know to much about metal work unfourtunately. What kind of hammer and dolly should I have, right now all I really have is a flat faced body hammer and a universal dolly. I'll look again where the curve is but it looks really nice.
Any other tips are welcome. Thanks, Adam
PS Peter, I'm sure it would be nice to have them now. Adam
01-09-2005, 07:37 PM
Are the roof braces bent? If the roof has been sat on or walked on the inner cross braces get bent. If a brace is bent, often straightening up the brace and working out the buckles at the roof edges will solve the oil can problem. Bob.
01-09-2005, 08:02 PM
The roof braces are straight. There are only two of them. at least I think they are straight I will look when I get home this weekend. Thanks for the thoughts. Adam
01-09-2005, 08:47 PM
It has been a lot of years sense I've repaired a Charger roof. As I recall the roof braces look like an upside down hat section. See if the sealer between them and the roof is loose or has missing sections. Run your fingers along the sides of the hat section to feel for small buckles, incase the braces have been damaged. If the braces were down or the sealer was missing or shot from age the roof would get floppy. Bob.
01-09-2005, 09:47 PM
I think I see what may have caused the problem. No offence but I see what I call "metal abuse". Deep gouges from a sander. I presume this was to remove the paint. :roll: Hard sanding builds up heat and can warp a panel by expansion.
That's besides the point. On to the fix.
If the low spot is tight (pushing on the underside does NOT give a pop like oil canning and rebounds), you need to do a little stretching to bring the low spot up. Start out inside the outer edge of the low spot and work around the perimeter (circle). Use light taps-On dolley. After the first round, check progress. If more is needed, move in an inch and make another round. Repeat as necessary to raise the low spot. If you start in the center, you'll just raise a ripple. (Like a rock in a pond.)
If the low spot is loose (oil cans), you need to shrink around the low spot. Start outside the rim of the low spot and work away from it. You can sometimes detect the location of "extra metal" by pressing on areas away from the low spot. It takes a "feel" for the metal though and has to be away from edge crowns. A good area will deflect and rebound with a little pressure. An area under compression (causing an oil canning effect in other areas) will feel a little tighter. Hard to explain unless you've done it. This is what I call a convergence. If you can find the spot(s), shrink in this area. More times than not, it will be two to six inches away from the low spot.
Next time, don't GRIND paint off cars. :D
01-10-2005, 04:36 AM
Next time, don't GRIND paint off cars. :D
Ken, What do you recommend for paint removal?
01-10-2005, 06:19 AM
To see what the panel is doing lay the ruler on its side and push down on the ends to make it follow the curvature of the roof. Do this front to back as well as side to side.
Templates front to back and side to side would helpful as well.
Do not worry about ruining your roof. If you have good metal to work with, it will forgive a few screw ups along the way.
If the low spots are raised, and smoothed to the proper contour and you are still getting the oilcan effect, then you need to shrink. As I said before, you can shrink from below with a shallow crowned disc to keep the roof from popping, but I don't think you will have to.
First take care of the low spots (not the big oil can low). A simple way to do this if you are having problems making the hammer and dolly do it for you is to lightly tap up from below with a dolly first then use a ballpeen hammer. You can use the ballpeen to stretch a small amount without hurting anything. You tap from below while watching from above. You can see every little nub that it makes. You will have to hammer and dolly to smooth the small stretched areas into a larger shallow bulge, then shrink with the disc. Do not be afraid to use the hammer and dolly. I really started to learn when I lost my fear of hurting the metal.
Practice this on a scrap part first to get a feel for it. Experienced metalshapers may cringe at this, but this is a great way to learn what the metal needs. You either have enough surface area, too much surface area, or it's just right. The metal is either smooth or it is not.
I would not hesitate to stretch past where it should be, and shrink below where it should be a dozen times without fear. You will get a feel for it.
01-10-2005, 04:42 PM
Ken, the roof was stripped with clean and stip discs. when I did it. They work great for removing paint and rust. The gouges you see are from whoever did it before. Like I said in my origional post someone had worked on it before, and put all the filler into so I assume they did it, and as you said, the heat from that could have caused. Unfoutunately now I have to fix it. :x But that's the way it goes.
Thanks for the tip on the hammer and dolly. I will try the hammer on and see if that works if the spots are low like you stated. I will try the tips you gave.
Thanks for the tips. I will definately try them out, maybe this weekend if things go well. I will try to practice on a scrap piece. Well maybe I can relax a little now about not ruining it.
I will let you know how I make out. Thanks, Adam
01-10-2005, 05:53 PM
Next time, don't GRIND paint off cars. :D
Ken, What do you recommend for paint removal?
Kerry, it depends on what's on the car. (Type of paint, how many coats, rust) My first choice is paint stripper. It's a little messy sometimes but gets down into creases, ridges and corners. It's great for removing old bondo too. And, it's garanteed to remove any silicones or wax. After brushing on the remover, I use a high pressure washer to take everything off. Follow that with drying and a coat of metal prep. Only a light sanding with a DA is necessary when your ready to work a panel.
The only time I've ever used a #36 disk was on a car that had about 20lbs. of glass-filled bondo in various places. It was the only thing that would cut through that stuff. I was dressed in a "space suit" with air supply and worked in the wash bay. I still itched for weeks after that. :D
More often, a DA with 80 grit will cut through the older lacquer paints pretty quick. The trick is not to work one spot too hard and keep the sander moving. Let the sander do the work. If you use "good" paper, you can strip a car with about 5-10 disks. I went through a whole roll of the cheap stuff once--It's not worth the savings.
Another good way is the air orbital sander with the water flood. Use 100 or 120 W/D and go at it. The water keeps the panel cool and helps keep the paper from loading up. I use this method for thick paints (Urethanes, enamels) or when a chem-strip is not possible. If you're working on aluminum panels, this is probably the best way to do it.
01-10-2005, 06:07 PM
Forgot to mention: Those "clean and strip" disks can generate a lot of heat too. One way to tell is if you see "smeared" or "galled" paint. That's a sure sign that you overheated a panel. :(
01-10-2005, 08:54 PM
Good point Ken, I use them on a Makita 7" Sander/Polisher, and run it a low speed. I haven't had any problem with any heat doing it that way on the projects I've done. If someone was useing something fairly high speed I could see it being a problem. Thanks, Adam
01-11-2005, 04:33 PM
You can bring the center of the oil canned section up by bead blasting it on one side. (The out side) Try this on a flat piece of sheet metal and you'll see it happen.
01-12-2005, 01:57 PM
Thanks for the tip, I'll have to give that a try. Thanks, Adam
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