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Discussion Starter #1
1970 Mach 1

A previous owner must have filled in this seam at some point, and painted over it. Now the surface paint is cracked and there is surface rust on the underside. This is located near the bottom of the back window and top side of the trunk.

Water gets in through this crack.

What would be the best way to fix this? Thanks!




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Man I've seen that on so many Mustangs and classic unibody cars, particularly if they're driven much. I thought about some type of reinforcing plate under that corner.
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The filler panel is welded in at each corner. The gap is then filled in with a seam sealer, but with a valley, not flush with the surface. Every Mustang where I have seen them try to fill this seam in, make it flush, they have cracked.
 

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Discussion Starter #4
Any ideas on the best way to fix it? The rest of the body and paint are in very good condition, so my hope is that this one location can be repaired (on both sides, but the other side is not leaking-yet)


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look at those seams from inside the trunk if that is possible and see if you can see anything odd somebody may have been trying to address


weld them solid, grind, prime and re-paint


grind all that out and see what is under there


all metal or panel bond would probably hold it(they don't set up totally rigid) as compared to what looks like regular filler somebody used


any solution is going to not be too much fun because what is there is going to have to come out


if that wasn't done very well there may be be some surface rust starting in those seams


there is also always the question of why somebody did what they did in there



just thinking out loud
 

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Short of grinding it out and redoing it there isn't an easy fix. I see that on more than just unit body cars, I think most of it is caused by people letting the trunk fly up when they open it.
 

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Discussion Starter #7
This picture might provide some additional info. It’s not too bad underneath, mainly surface rust. Water does get through that seam though and it looks like rust is in there under the cracked paint on the seam. Why the seam was filled is unknown but most likely to address rust in the seam.

In this picture my finger is pointing at the underside of the seam.



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I just looked at this again and I wondered if there isn't something like an expoxy or something that can be injected from underneath to help seal it up...better yet a reinforcement cold-welded then the seam welded underneath...then a pen-tip paint application done above with blending that may significantly reduce the cracked paint appearance. I'm just thinking aloud...
 

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I'd also be concerned at what lies hidden in that corner under the Window trim and seal. Its a frequent spot to find rust through.

 

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1966 Mustang Hardtop 289 4 Speed
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If I had to change another quarter panel I’d panel bond that seam or weld it up solid. It’s pretty much impossible to stop a new repair from cracking again otherwise.


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Discussion Starter #11
If I had to change another quarter panel I’d panel bond that seam or weld it up solid. It’s pretty much impossible to stop a new repair from cracking again otherwise.


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Hey thanks for the additional info here. I had the car in the shop recently, actually, and had the entire rear window removed, new seals etc. The PO used some sort of caulk or improper sealer around the back window. Was a mess. Now it's fixed correctly.The window and weather seal on the trunk lid both leaked and were fixed initially, and we didn't realize there was a third leak until those were fixed first.

The shop grinded out the seals in the picture, filled them with some other type of stuff that doesn't get as hard as the Bondo that was in there (not sure exactly what it was), and then repainted the area. I think it's fixed correctly now. I can say that there are zero leaks in the trunk now. I even put paper towels all over and hosed it down - not a single drop!
 

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What about brazing that seam and then grinding it to look like the original seam ? I dont like the idea of just welding the seam and grinding it flush. That seam is part of the originality to me.
 

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Discussion Starter #14
Here is how it looks now after getting it repaired. It was done with a two part seam sealer that has both a bonding agent and sealer properties, according to the shop.

It does still have the original “seam” look. Here’s a pic:




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IMHO, you fix it "right". That means pulling the rear glass, exposing the seam in its entirety and repairing the damage. If rust has significantly affected the thickness of the metal then it needs to be cut out and repaired. Even if you weld the seam it's STILL a seam and will eventually crack...maybe not ON the weld but along either side of the weld. Most of the time that I have seen the original lead crack it's because the deck lid is adjusted too far "down" onto the trunk seal flange on the window filler panel which causes a binding and flexing of the area. If you don't feel like leading the seam again then I'd use Evercoat Metal2Metal as a substitute. Sealing the back side with epoxy primer will prevent rust from starting from that side. I'd also use a liberal application of 3M Bedding and Glazing Compound along the bottom edge of the rear glass, between the gasket and filler panel to prevent water from standing along there and in the corners, under the trim.
 

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Discussion Starter #16
Thanks Woodchuck. To be honest I don’t understand all the details of what you described here, but before this was done the rear window was actually completely removed and resealed. Also while it was out the headliner was redone as well.


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Any ideas on the best way to fix it? The rest of the body and paint are in very good condition, so my hope is that this one location can be repaired (on both sides, but the other side is not leaking-yet)
The filler panel is welded in at each corner. The gap is then filled in with a seam sealer, but with a valley, not flush with the surface. Every Mustang where I have seen them try to fill this seam in, make it flush, they have cracked.
69GT350H is correct. I have seen this before. Mask off the paint on either side of the seam, and remove the improper material from the seam. Apply Seam Sealer with the correct slight dip in the seam, and repaint the seam. Properly done, it should last for years even in daily use.
 

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Discussion Starter #18
69GT350H is correct. I have seen this before. Mask off the paint on either side of the seam, and remove the improper material from the seam. Apply Seam Sealer with the correct slight dip in the seam, and repaint the seam. Properly done, it should last for years even in daily use.
Okay then it was done correctly because a seam sealer was used and it has the slight dip.


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Here is how it looks now after getting it repaired. It was done with a two part seam sealer that has both a bonding agent and sealer properties, according to the shop.

It does still have the original “seam” look. Here’s a pic:




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Much better! Well done.
 
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