Roof patch causing warp / oil canning

[HAULIN' IT]

I'll add a bit more to the discussion.
First a question for Robert...Was the tailgate you showed in the video for sure mig welded solid? Ive seen some things like that before (glued, Lenco spotwelded along the edge & some tacked every couple inches that you could see EVERY weld & gap), but never a fully welded seam to that degree of ghosting. What I have seen several times (which if it was done on a big long seam like that, I can imagine it looking very similar) is the weld has a side (or both sides) with a real bad "undercut" left along the weld. An example would be a guy closes up the gas filler hole in the truck cab corner by laying a patch in behind & welding around the hole. Once finished, the INSIDE edge of the weld shows the "step" after some time in the sun because of the drop-off...the outside does not because it was able to be easily ground smooth with the original metal. One very good reason for your insistence of the butt-welding panels. Like you mention, you go to a car event on a hot day &/or as the sun is going down & dew is starting to set in...you see some interesting things!

To add to what Robert said about the question about how/why the ghosting...at least with using panel bond, I think there is another factor, but really all the same reason Robert stated & that is, simply the difference in the makeup of the items heating, cooling, ect at different paces & react differently to heat, cool cycles...we have a sharp cut metal edge of the original panel, we have a sharp cut metal edge of the patch...in between/around the edge is a very good insulating item (somewhat like a plastic material). See where Im going?

So what to do to minimize the ghosting? I have found a few things that go the right direction to minimizing the chances. First, you taper off the upper square edge of the original metal & the patch...similar to what doing drywall would look like. Second, As I mentioned earlier, I prefer the Fusor 208. It dries harder & is more solid than the "panel adhesive" products like 108, 110 ect. which is really more a metal glue intended for flanges which stays slightly "gummy" & can never really be feathered out.
This part is going to make Robert go take a blood pressure pill, but as I stated earlier...Im a realist. Body filler is really a friend to nearly everyone until they hone skills in like Robert has. So here it goes...Don't try to make it "too good". Yep I said it! What Im getting at is this. I promise you if you work real hard & make your pieces real close to flush not needing/having much room for "glop on top of it" it WILL ghost line on you. If you have room for a uniform coat of hard base filler like Duraglas, all-metal, something of "insulation" on top of your patch, the odds are much reduced you see a line. Just like nsb stated, a polyester primer is also my choice (Clausen Sandy or Rust Defender is my pick). I distinctly remember one of the Roofs I did years ago on an Impala. The local department had a car & wanted to make it an undercover detective's car. It started out Black & White with lights, antennas, ect. & ended up all White with pinstripes, mudflaps & full wheel covers...looked just like grand paps ride! I grabbed a pick hammer, tapped down the mess the last guy made drilling & mounting the stuff & glued plates over a couple holes on the roof, one on the trunk & an oval on the post from the spot light. That car was back in the shop for a deer hit & someone backed into it years later...couldn't find any of those spots, not a flaw to be found years later. It sat outside everyday, but it was White also. What Im getting at is: "The Roof was ruined by the guy with the drill", I made it better even if it would have shown a ghostline...no one but a trained eye was going to find one if it was there. One of the neat things of over 30 years "practicing" auto body repair...you get to try/see how things work out or dont work out. Lorne