Weld through primer

[Mark Wilson]

I've a feeling that I'm about to be shouted at, but I' m starting to question whether I should be obsessively priming before welding, as I have up till now. I was consulting NeilMG's resto thread, yet again, when I came across a suggestion from him that in the context of plug welds weld-through priming could be a hindrance. I recently had a problem getting some plug welds to penetrate properly until I removed the primer.

I'm about to do some lapped floorpan joints with plugs followed by seams on both edges. (I know, but that's my choice and it isn't what this post is about). I will revert to being obsessive when it comes to etch priming, seam sealing and stonechip, but I'm tempted to omit the priming inside the lap. My reasoning is that the factory didn't use weld through, that the welds will be better, and that there will be virtually nothing left of the primer anyway.

Am I about to be burnt at the stake as a heretic?

Mark

[les]

No, you're not alone, it can be obsessive, better than saying 'That'll do' though! You could carry on with the weld through but scrape off the area you want to weld. At least you will have some benefit. I have found this easier to do with spots as you can simply run a scriber around the holes, cleaning the metal underneath. With a lap joint you only need a line of clean metal to take the initial spark, then the primer will burn.

[don58van]

Thanks for asking the question, Mark -- I have been wondering about this too.

I am using a popular brand of copper weld-through, but I am not sure what tradeoffs I am making. I am using spot, plug and seam welds.

Any other views on this?

Don

[Roger46]

Is weld through primer for MIG welding? I thought it was really designed for spot welding and I've also always had problems trying to MIG weld through it!

[don58van]

I believe that is is primarily for spot welding.

However, it presumably is formulated to be highly resistant to heat (as well as being conductive). It therefore may provide a durable coating on surfaces that are butted or lapped and plug or seam welded.

Don

[Redmoggy]

I've not had a problem with Aluminium or Copper weld through primers. Have you tried turning the power up on the MIG? Remember when you plug weld you want to form a liquid puddle in the middle before then moving the torch outwards in a circle to form a finished puddle around 8-10mm that is relatively flat. The panel should idealy be clamped either side of the plug weld.

Regards
Rod

[Mark Wilson]

Thanks everyone. Redmoggy's technique is pretty much what I do, and has mostly worked OK with the primer in place. But I was not well pleased when I realised I had a couple of bad'uns and had to drill out and start again. Might just have not taken enough care with the settings, but when I saw NeilMG had doubts about priming plugs it did get me wondering. Think I'll carry on priming but clean the centres and edges as Les suggests.

Mark