Seam Weld or Lap Weld

[cobolman]

I've been practicing welding a LOT before committing to doing important work on my car and there's one question that keeps going through my head and thought I'd air it here for opinions.

The classic mini was, as you all know, constructed using a multiplicity of spot welded lap joints, because its cheap and required little skill in construction

The current wisdom in welding says to use seam welds, to avoid trapping moisture in the lap

When restoring a car, the approach seems to be to replicate spot welds, where they were originally used. But if letting in a patch, use seam welding

But, there's a catch. Seam welding is difficult.
- It's easy to leave pin holes through which moisture can strike, and;
- Burning through thin car metal is a regular occurrence that may well require ugly 'filler' welds (ask me how I know!)
- Grinding down welds, without grinding through original panels, isn't easy

So here's my question: Why do we restorers hold ourselves to a higher standard than the original constructors?

Bonus question: Do you use lap welds or seam welds?

Extra bonus question: What seam sealer do you use (I'm aware that's an obvious way to address the pinhole problem in seam welds)

Thanks in advance

[abs]

Seam welds with close to a 1mm gap between panels so as you weld it pulls the panels together thus avoiding distortion. I also have a block of copper as backer if the gap is a little large and of course magnets to hold the panels together but they don`t last long due to the heat.
You also need to keep welding in different areas to keep the heat down, sorry if that`s obvious to you
Using this method you cannot actually see or find the repair.

As for everything else, spot welds where they should be and plug welds where the arms of the spot welder won`t reach.

[cobolman]

Seam welds with close to a 1mm gap between panels so as you weld it pulls the panels together thus avoiding distortion.

Thanks for sharing your thoughts, abs

I see this same advice in lots of places and have tried it. With new steel to new steel, it works a treat and I can repeat it ad nauseum.

With new steel to frilly, thin, 65 year old original steel, it's a recipe for blowthroughs of the hurricane variety

The difference between a good weld and a blow through needs timing of such finesse, it's like feathering the torch trigger and knowing the difference between 45ms and 50ms (for this learner, anyway)

I've tried using a copper backer too but with varied success. Trying to get it to stay in position requires either the flexibility of a cirque de soleil gymnast or magnets that are so strong they would destroy a computer at 100 paces (hyperbole mode disengaged)

I probably sound frustrated (probably because I am and maybe I'll go through yet another large tank of hobbyweld gas as I practice to get better

I keep hoping there's a 'silver bullet' that will help everything drop into place

[abs]

Ah that`s the other thing, throw away the hobby gas and get a bottle of Argo shield.
Argon is what you need so no wonder you`re having a hard time !
Also keep the amps down and push towards the weld bead as opposed to running away from it. When you get it right it`s very rewarding. The underside of the weld should match the top. Do a small section at a time and join them all together eventually.
If your original steel is not the best it`s time to cut a little more out or it will bite you in the ass a few years down the road.
I sometimes use 1.6mm sheet as I can keep the heat on the thicker steel in places where you wouldn`t be able to tell and it keeps distortion down.
A new inner sill to the original floor pans being one place as it`s an easy panel to fold up in the shop
Good luck, we all started off the in the same place

[Tupers]

Just a small bit of pedantry.
One piece of metal welded on top of another is a lap joint.
Two pieces of metal welded together on the same plane is a butt joint.
A seam weld is a continuous weld which out be carried out on a lap or butt joint.

If you’re experiencing pin holes in you welds you likely have issues with shielding gas or you haven’t removed all of the contaminants and rust from the piece you’re welding.

I’ve been on an Austin 7 recently with both MIG and TIG and although MIG is a lot more tolerant of grime you really need to be working with shinny steel.

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[Polarsilver]

Just my opinion having been in my past working life very aware of Repair Welds that were needing to be carried out by Certified Welders .. also spent some years within Pressed Steel Cowley where Bodyshells were made .. maybe this has influence on my thoughts when i look at any hobby repaired Bodyshells.. i ask myself will that repair welding withstand in the event of an unfortunate crash Impact to that Bodyshell.?

I cannot weld to that needed standard ,so for me i would go to the "expert" ( not a Bodger ) who has the knowledge + skill & equipment.. yes that costs but gives peace of mind & come time to sell i would expect to recover some of those repair costs as the next owner also gets peace of mind that the cars structure is sound.
Each to their Own.

[cobolman]

Just a small bit of pedantry. ...

Thank you for clarifying. It's a new world to me and I enjoy learning

I’ve been on an Austin 7 recently with both MIG and TIG and although MIG is a lot more tolerant of grime you really need to be working with shinny steel.

Good reminder about the grime.

That's some really good looking welding!

[cobolman]

... throw away the hobby gas and get a bottle of Argo shield.

I scuttled off to check that out but note that the gas constituents are pretty much the same. I'm using HobbyWeld 5.

If your original steel is not the best it`s time to cut a little more out or it will bite you in the ass a few years down the road.

Very good point!

Good luck, we all started off the in the same place

Thanks

[cobolman]

Just my opinion having been in my past working life very aware of Repair Welds that were needing to be carried out by Certified Welders ..

You make some really important points.