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When doing a butt weld joint, is it best to leave a small gap for the molten metal to fill, or is it best to get the pieces edge to edge with no gap. I'm going to be installing a lower rear fender patch soon, and I just want to make sure I do it right.


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Thin metal....? Close a fit as possible...

I can butt weld material with no gap up to 1/4" or so with my MIG....

After that I bevel the edges and leave a 1/16" gap or so....

Leave a gap in thin material and you're asking for burn-through, IME...

Pat
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People are continously talking about leaving a gap when mig welding sheet metal, and it causes nothing but trouble anytime I've tried it. Maybe it's just trickled down from plate welding techniques. I've been able to weld up thin gaps, but after a 1/32 or so you have to sort of zig zag to keep from blowing through. I try to butt them up flush now and it makes it a lot easier.



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I suggested the lap weld technique to the gang a while back but they don't like it because of the possibility of rust starting in the overlap area...

Personally, I've never had any problems but hey, it's their car..*G*

Pat
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I put my floor pans in using butt welds. In some places, I ended up with probably some 1/8" gaps, but I did what appears be called stich welding, and did small sections at a time until they were all filled in.

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[color:blue]No gap! Air is not easy to weld!</font color=blue>

Weld by tacking several spots first, then between the tacks, until you have a continous weld. Don't "hold down the trigger", use intermittent welds to hold the heat down and control burn thru.

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When overlapping the weld use a Weld thru primer - the one I use is made by 3M -it is made to prevent rust between welded panels.
 
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