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Feeling a bit confused on whether to use this or not between panels for spot welding.

The idea of having the zinc there to provide cathodic protection seems good, especially when you see how quickly metal goes rusty after it has been welded. Having repairs rust from the inside out doesn't seem particularly appealing.

Some don't appear to like it though and believe it produces a poorer quality weld when used. Has anyone experienced that?

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If you are talking about true resistance spot welding, using a "weld through" primer in the joint is endorsed by many, but not all, of the current car manufacturers.  I haven't seen any problems using it.

Ed

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Remember weld thru primer comes in zinc and copper.

A friend who was a bodyshop man for 50 years reckons one is better for mig, the other for proper spot welds. Also it has poor adhesion, and the way it works is misunderstood. I "think" it sort of melts a bit and seals around the weld. But it isn't a general primer, and it is only part of the rust protection system. 

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Don’t do much spot welding......

I use Upol #2 weld through elsewhere and it seems to work. In fact any zinc rich primer seemed to weld through ok. Apart from the Toolstation stuff, which is more reluctant.

IIRC, you can also get weld through seam sealers specifically intended for spot welding seams.

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I'd be happy with a gooey zincky mess :biggrin:. I don't really worry about unpainted seams so much that haven't been touched by the welder, as those will be seam sealed or have cavity wax on top of them. It's just the immediate area of the weld itself that concerns me as it'd be so frustrating to have it bubble at the seam after it's back on the road (as some previous repairs on the car have done).

I wonder which is better for which type of weld. I'd guess copper for spot and zinc for MIG?

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I've only used zinc, but notice that for plug welds, the primer still acts as a contaminant.  I get better results if I clean primer from the weld hole. It wouldn't survive in that immediate area anyway.

Ed

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1 hour ago, ed_h said:

I've only used zinc, but notice that for plug welds, the primer still acts as a contaminant.  I get better results if I clean primer from the weld hole. It wouldn't survive in that immediate area anyway.

Ed

I do the same,easiest with a flattened off drill bit.

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