Car bodywork welding: Is that enough penetration for a panel repair?

Question

I'm practicing welding with the purpose of doing some bodywork on my own car soon, and lately I've been working with flux cored MIG (gasless MIG).

I've some scrap 1.25mm (18 gauge) steel sheet, which I'm using to practice. First time I did some test runs trying to find out some appropriate settings, and for that thickness I found out amperage between 55A-60A and wire feed between 2-3 gives a good result for flat beads. I've been able to get decent beads with very good penetration (to the point of making a protrusion to the other side) with those settings. On this plate I was experimenting with amperage/wire feed, but you can see most of the beads went through pretty well:

When I try to weld a butt-joint however, for some reason the same settings don't seem to penetrate as much. I've simulated a small panel repair, and I'm still able to see the different plates edges on the other side. I thought the spot welds would go through and cover those lines. I don't think I held the MIG gun trigger shorter than I did on the solid sheet tests.

Is there any reason the butt-joint spot weld doesn't penetrate as much? Should I increase the settings for the joints or maybe hold the trigger longer? Is that enough penetration for a car bodywork repair?

Answer 1

Penetration is only a key point if you are welding structurally. For body panels you are just trying to first stitch it together (as you are showing in the pictures), but then you need to fill in between the stitches so the weld is complete all the way around. Then, once you grind it down, you have a complete seem without gaps. The "stitching" process is a means by which you are adding weld without getting any single area too hot. You stitch (add a small weld area) on one side, then move to a cold area and do the same thing. Then once you've added several small welds and the first area has cooled down, add another weld close to, but not next to the first area. Keep rotating it around until you have the entire area filled with weld. If after you've ground down the area (after the stitching is complete) you find small holes (like in your image), go back and hit those with a few small stitches and regrind.

Overall, your stitching itself looks fine. You really just need to fill in the holes at this point and regrind. Then you'll be golden.

Also, the welded area doesn't have to be perfect after the weld/grind process, as long as you don't have any holes. If there are a few low spots, this will be taken care of during with a skim coat of body filler when you're doing your body work.

Answer 2

Did you keep moving the trigger on/off/on/off/on ...... to get the rows of spots ? If not , I expect that is "pulsed MIG". The machine keeps cycling the power up and down rapidly. The work material is so thin it is difficult to have a low enough power level to prevent burn through . But at the same time there must be enough power to maintain the arc. So the machine changes the power up and down to give a good "average" power.