[Supercharged03]

I know there are a lot of opinions about sheet metal welds and tig vs. mig. I thought I'd show some of my results.
I only have a mig machine - Miller 135. I run .023 wire in it. I previously used a different Miller machine I'd borrowed from my son (a 140 I think) and used .030 in it.
When I started on my project, he told me to just build up tacks - skip an inch or two between the initial tacks and build on each one in sequence. I've had fair success with this method, minimizing warpage on some long seams.
Some folks say this method isn't welding, that it leaves too many voids on the back side of the surface.
I took a couple of photos of the back side of my latest work. You tell me - do these built-up tacks look sufficient?

 

[idrivejunk]

Not quite. With MIG, a gap between panels of approximately 1.5X the metal thickness is a proper setup.

 

[alwill923]

looks pretty good to me. There is a fine line between full penetration and burning a hole. I would say 90% is good and I would not worry about the 10% as there is still penetration.

 

[123pugsy]

If you see a seam between panels on the back, it's not fully welded. Knowing that your panels will need stretching, you want full penetration. MIG welds are harder than TIG and may crack if over worked. Please let us know if they do crack. Curious just how much they can take.
There's nothing stopping you from tacking the unwelded seams showing in your pics from the back side.

 

[Supercharged03]

True, pugsy. I do tack the unwelded seams on the back side of the pieces I can access. I also backlight the work with a flashlight to spot pinholes. I typically don't work the welded pieces much after they are welded, so I haven't observed many cracks.

Here are a couple of photos of the finished welds (is anything ever "finished"?)

 

[123pugsy]

If there are no holes, then it's "welded".
They are strong for sure if you go back and spot tack any places missed.

 

[idrivejunk]

This ^^^

Grind first though. Recheck with light after any grinding. When its all done, probe around the joint with a pick or slow drill. Because the cracking often percieved as a fault in MIG welds is just where the sheetmetal is too thin right next to a weld where fitment was less than fully flush. Thats my findings anyway.

 

[Supercharged03]

This is good news. I feel better after this discussion. I had fears of driving down the road and having pieces of the fenders and body breaking loose and flying away in the wind.

 

[ChevyHound]

Looks like you got good penetration.
On the surface you have some porosity maybe play with your gas a bit.
Ideally you should run it with a gap. Use a pieces of MIG wire to gap it. This will allow you to burn through without too much heat.
When tacking , stagger your tacks. One middle , on far , one near and so on until you are comfortable with the weld not "pulling".
Then stitch your tacks half inch at a time.
Middle , far , near. Allow for cooling in between "stitches".
Look up Pulse MIG. It'll give you an idea of the method.
Looks like that 135 doesn't have Pulse MIG , but doesn't matter you can pull the trigger yourself.
Do not pulse mig with fluxcore btw.
Not that you can't , just that you have to clean every tack every time.