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Old 04-07-2021, 07:16 AM   #24
MP&C
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Join Date: Jan 2013
Location: Leonardtown, MD
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Re: Roof patch causing warp / oil canning

Quote:
Originally Posted by ItWillBeSlow View Post
- Can anyone elaborate more on the ghost lines, and why these are a risk? I planned to sand/grind all the joints, skim coat with bondo, epoxy, high build, then paint. Im not understanding how this patch would be different after it is prepped like the rest of the cab.



This video shows a reflection in the rear tailgate of a 54 wagon, the owner had done a nice job of restoring it, the car was neat as a pin. The only flaw was that he had used a stepped/flanged repair on the patch used on the lower tailgate skin. You can see the exact location of the seam, despite the filler used, and you can even pick out the screw holes that were plug welded closed. The other patches on the car were all butt welded with no discernable ghosting. I was given permission to take and show the video in hopes that others could learn from it and take prevention steps.


https://www.youtube.com/watch?v=OGhFEfVqxb0


This occurs because you have two thicknesses of metal to one side of the weld, and a single thickness to the other side. Two thicknesses will take longer to heat up when exposed to sunlight (think car show) and longer to cool down that evening than the single thickness on the other side of the weld. This differing expansion and contraction rate will cause a ghost line right at the seam, showing exactly where the repair was made. So when I respond to various patch panel welding questions, this is also why I stress to grind welds on both front AND rear of the panel. We need the panel thickness consistent to help eliminate the cause of ghosting.
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Robert

Last edited by MP&C; 04-07-2021 at 08:25 AM.
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