And then the outer join would suffer corrosion [emoji6]
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It an overlay panel...
The defender material is very thin, you'd need to be a gun on the TIG at some low amperages and ideally pulsed to get acceptable work without blowing holes and/or warping it to buggary.
As for the corrosion, much of this would have to do with the sugaring of the weld on the backside of the weld pool where there is no gas coverage. Also, an out of position, overhead TIG weld on that material would be... um... interesting for a lot of welders out there.
I had cracks on an old ally boat welded and it fixed the cracks But new cracks developed at the weld.[bigsad]
I have read that the best way to seal the roof joints in a Defender is to remove the interior lining and seal it from the inside with a waterproof sealer.
I haven't done that because my leak is only minor and I'm not sure it's actually from the roof joint.
Cold cracking occurs in the HAZ (heat affected zone). Particularly prvalent with aluminium alloys.
https://youtu.be/0Q4ZeN9-v-k
Also, check out the distortion on the job at the 40s mark, and the 2m50s mark. This bloke is a gun, knows his stuff, and has top shelf gear.
My skill level is well and truly in the sikaflex end of the spectrum!