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Tried a patch with no gap, seam buckling in

So I tried welding my first patch panel with zero gap (.023 mig with C25). Alignment was pretty much perfect. But even going as slow as I did...meaning like a few tacks whenever I could get to it over a few days - It seems the seam buckled in a bit.

I did hammer and dolly mostly all of the tacks a little. I dont think it could of possibly been 100% heat induced as I never did more than a few tacks several inches apart from each other before letting it cool.

I will post pics later tonight - but since solid mig wire (er70 is it) is so hard to hammer - is there any chance at saving this preventing the use of a bunch of filler? I am nervous I am going to just stretch all the surrounding metal and make it worse.
I started a topic on the exact same issue, long story short flatten your tacks on both sides (if you can access backside) then planish each weld. Keep tacking, flattening, and planishing until it's finished. The metal is shrinking from the heat of the weld (yes, even a small tack) and causing it to pull in. Tig welding is more malleable and easy to planish/bring back if you have access, otherwise Mig works but just takes longer.
What area on the car (?) are you patching? Most automotive panels will have an outward crown to help hold their shape. Looking at a cross section view, it would appear to look like an arc. When your welds cool off, they shrink, so this "arc" along the weld seam tends to make a flatter arc as the shrinking occurs. That is what is causing the "buckled in" look. You need sufficient planishing (hammer and dolly) to stretch the area of the weld and HAZ. The other point I would make is if you're having difficulty in getting the panel to raise up to where it belongs, perhaps using the hammer on the inside and dolly on the outside while planishing will better help to persuade the panel outward, giving you better results. But just as you check a dolly to insure it fits the panel, you wouldn't want a flat hammer on the inside as you will likely leave coin marks. A slightly crowned hammer where the edge does not touch the panel would be needed..
Sorry for the delay guys! I was dumb and did minimal planishing during the tacks. I ground them down and moved on. So now I have that valley to deal with.

I am currently working on the area in front of the rear tire on a 2004 dodge ram pickup bed. That original was banged up so wanted to replace it instead of trying to straighten it all out. Got a new panel and fitting it up nice. I am trying to planish it now, but as I've found like you guys say - it must be much easier when you go tack by tack. This was me just being eager to get started and not reading enough. It is pretty much all tacked up. I am going to try the dolly on the outside and really try and bang it out. It isnt more than a few millimeters or so. Def under an 1/8 of a valley. In some sections its almost flush...so at least some of it looks good.

I dont have any image hosting account so cant post pics, or else I would.

Once I get this panel right, I am going to begin tacking up the new wheel arches which are already cut out. But I want to dial my skills in a little better because I have a feeling that area is going to be more difficult.

PS = this area actually has an inward crown horizontally, and a outward crown vertically.
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It isnt more than a few millimeters or so. Def under an 1/8 of a valley. In some sections its almost flush...so at least some of it looks good.
Its probably a little deceiving because the shrinking is pulling on the surrounding metal, so you may be right about the valley, but the area around the valley is low also. When you planish the weld, you need to apply firm pressure on the dolly like it is trying to push the metal back up, and the hammer stretching is just allowing the dolly to push it up. Other wise you could be pounding the metal down lower.
Your also dealing with thinner metal then what older cars were made with which adds to the problem. Newer cars metal can give you some real fits getting it to behave. Bang around too much and you will have a real problem on your hands, know when to stop before you get to that point.


Trying to be the best me, I can be
If you are intent on metal finishing (or getting it very close) then using a MIG is gonna be frustrating. As you have found the welds are very hard and do not want to planish out very easily. On late model stuff my opinion is a little filler doesn't really matter. As long as you can keep it 1/8-3/16 that is gonna be fine, especially if you epoxy then apply filler. Do the best you can with where you skill set is now. As you gain knowledge apply it and and your work will start to show it. Nobody was born being able to do perfect metalwork it's a skill that takes time, tools and the proper methods. If you want to learn from a Master, check out https://www.tinmantech.com. He's got some excellent videos that you can rent instead of purchasing if your funds are tight. Money well spent.
Mig welding
Isn't my favorite way to weld Thin sheet metal body panels BUT the industry as a whole accepts it.
It leaves a Rock hard bead which is rock hard and almost impossible to Plannish out without cracking as the Bead is harder than surrounding metal.

Tig welding
Is Much better for thin sheet metal it leaves a softer bead wich can be planished out much easier without cracking and has a much smaller heat affected zone.

Gas welding or Hammer welding
Does have a Larger heat affected zone than Tig welding.
The bead is Plannished while the bead is red hot and worked to a smooth finish so no grinding is generally required when done properly.

There are many many different ways to Weld metal, no one process is perfect for all situations.
Just like there are many many different types of filler wire and Gasses and gas mixes.
Knowing multiple Processes having the tooling and experience to know which one to use when.
Knowing what type of filler material to use? and which Gas to use? again there are many that will work some are better and easier to use than others knowing which works best in you and your situation is up to you.