E Body Roof To Quarter Seam Problem

I'm doing the paint and bodywork on a customers 71 'cuda and am having a problem with the filler in the "C" pillar swelling. Ill start by saying I've been doing bodywork for a living for 40 years and this is the fourth rotisserie Mopar restoration I've done this year so I do know what I'm doing. Here's what has transpired so far. It's a 71 'cuda that still has both original quarters on it. The car was media blasted with walnut shells removing all the factory filler. I cleaned it ground it and wire brushed it clean. I filled the seam with "All Metal" which I use all the time for this and have never had a problem. It probably cured for 3-4 weeks while I was working on the rest of the car before painting it. The day after painting it I put it out in the sun to cure before sand & buff. I'm in Phoenix so it heated the cars surface to about 140 degrees. The heat made the filler swell on both sides enough to see a perfect outline of the seam. Take it out of the sun and it goes back down to normal. I called my paint rep and he got the rep from USC (The company that makes All Metal) and he came out and looked. Since the filler was softening in the heat the only thing he came up with was it was slightly under catylized. I ground it all back to bare metal and redid it using alot more hardner. Well it did the same thing again. The rep came back out and called the companies chemist and nobody had a solution. They are working on it but in the meantime since he knew I had to get the car done he suggested I use a differnt product. This time I used 3M's Mar Glass which is a fiberglass reinforced filler. Back down to bare metal again, redid it primed it and used my heat lamp on it to heat it up to 140 degrees and it did the exact same thing. Now on the fourth try I welded the bottom of the seam below the spot welds solid. I thought maybe something was leeching out of the seam and this would stop it. So after welding it I filled it with the Mar Glass again and primed it. Heated it up to 140 and it didn't swell. Raised it to 175 and it still looked good so I figured I got it fixed. Painted it again thursday looked great. Put it out in the sun today and even though it only got up to 110 degrees it swelled up just like before. Everyone who is involved including myself is totally baffled by this. I am out of ideas. Monday I will try to get the 3M reps involved but right now I'm hoping maybe someone here has experianced the same problem and has a solution. I apoligize for the length of thie post but I really ned to find an answer soon.

I don't play that game. I'd weld the seam solid and weld a full strip to close that dip or completely cut that lap joint out and replace it with a strip butt welded,use TIG welder

If you are set on keeping that lap joint original ,I can only suggest Marine-tex. I never used it on C-pillars, but have used it grafting metal to metal, fiberglass to metal,plastic to metal with amazing results,it beats the two-part metal bonding adhesive.

I'm assuming you're not applying filler over metal that was wirebrushed.

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I don't play that game. I'd weld the seam solid and weld a full strip to close that dip or completely cut that lap joint out and replace it with a strip butt welded,use TIG welder

If you are set on keeping that lap joint original ,I can only suggest Marine-tex. I never used it on C-pillars, but have used it grafting metal to metal, fiberglass to metal,plastic to metal with amazing results,it beats the two-part metal bonding adhesive.

I'm assuming you're not applying filler over metal that was wirebrushed.

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I can see how welding the seam solid would be a good idea but that doesn't explain why after doing the same procedure the same way 30-40 times over the years this time it doesn't work. What's the issue with using filler over wire brushing? Sometimes it's the only way to clean the metal and again I don't think I've had any filler related problems before this one. I do really appreciate the input.