I rarely MIG weld sheetmetal now that I have purchased a TIG, but there are many DIYers out there still using MIG welders to join automotive sheetmetal. This tutorial should give you valuable information on how to properly butt weld sheetmetal with a MIG welder.
The botched previous repair has been removed and the new panel tack welded.
Here is a close up so you see that the butt joint is very tight. Also notice that I've chamfered both the replacement panel and the original fender. This is a tip from Wray Schelin that proves to work great. No problem with burning through and penetration is excellent.
You have to weld thin gauge sheetmetal much differently than you would heavier plate, even in the 1/8" range. You must weld a series of spot welds, with each one overlapping the last. I try to overlap by a 1/3 to 1/2 on each successive weld. You will also need to stop at regular intervals (not more than an inch) and stretch that area out before moving on, otherwise, it will have more distortion than you care to deal with. You start by placing tack welds at 1" intervals to keep the two pieces in good alignment. You can then start the welding process. I prefer to start in the center of the seam and work out both directions toward the edge. You must stop and work the weld as you go though. To best do this, a 1/16" cut-off wheel works great for knocking down the proud weld bead. You want to leave just a tiny amount of weld above the surface, because you're going to work this down some as you hammer the weld to stretch the metal back out. You only work within the Heat Affected Zone or HAZ as we often refer to. This is the blued area around the weld. Do not leave the area when stretching a weld. Even though the surrounding area is distorted, it is still unharmed. It's just sucked in down some when the metal shrunk along the HAZ. It will pop right back into place when you stretch the weld seam. You stretch it by hammering on dolly, in other words, you place a dolly on the backside of your weld and hit it with a hammer, making sure your hammer blows are against the dolly (or post dolly if you have one) This will rapidly stretch the area drawn in by the heat and relieve the panel of the stress caused by the heat, removing the warpage. You can now add another series of adjoining spots and continue the process, jumping back and forth from side to side, working ever closer to the edges of the panel until finished. Once you've gotten the weld completed, you can go back and fine tune the weld seam work the panel with a slapper and dolly, producing a very smooth panel. I prefer to use a file to work down the final few thousandths of proud weld, rather than a grinder that will remove too much material. A shrinking disc will come in real handy too, to shrink any areas that you overstretch.
This is the right rear fender for a '39 Ford Sedan. It had previously been repaired (butchered) by another so called bodyman. Also pictured are the 19ga. repair panels I shaped for the repair.
The botched previous repair has been removed and the new panel tack welded.
Here is a close up so you see that the butt joint is very tight. Also notice that I've chamfered both the replacement panel and the original fender. This is a tip from Wray Schelin that proves to work great. No problem with burning through and penetration is excellent.
I try to overlap each successive spot weld by 1/3-1/2. This picture doesn't show it as well as I had hoped, but I think you can make it out if you look closely.
Here is the weld seam after running a complete series of 1 inch overlapping spot weld segments, followed by stretching along the Heat Affected Zone.
Once I have it roughed in on the exterior side, I remove the proud weld bead from the inner side and further planish the weld seam with a slapper and dolly, carefully bringing up the low spots and filing away the remaining few thousandths of proud weld bead. I then run over the entire repaired area with a shrinking disc to remove any overstretched spots. Here are the results. No body filler will be needed, the panel is stress free, and there will be no worry of seeing a seam or early paint failure, common to lapped repair procedures.
Randy Ferguson
