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?