That is definitely MIG and not flux core just looking at it. If you had to turn on a bottle before welding that is MIG. If you have to chip or brush off any slag that is flux cored wire.
I prefer to do a series of overlapping tacks, which means setting the machine a bit hotter. a one second tack, two to three seconds to allow the material to cool...lather, rinse, repeat.
It should look a bit like a stack of dimes when you're done.
The key is to not move the gun away when you let go of the trigger. An auto darkening helmet makes this a bunch easier as you won't be so likely to stray away from the seam.
Penetration on sheet guage is almost never the problem...the usual problem is burning through. The second problem is warping the panel. To mitigate the warpage problem it's best to weld 1/2"-1" in one area, then move to another part that is a little ways away and do 1/2"-1" there. Keep moving around and try to avoid overheating any one area.
When grinding the welds down go easy...it doesn't take much to put even more heat into the panel with the grinder if you're pushing it too hard. A 36 or 50 grit sanding disc and light pressure should remove most of the weld without too much heat build-up. If you see the steel turning blue you're being too aggressive.
As an aside the largest wire I've used is 3/32" Flux core with a CO2 shield gas. Big enough to burn through 1/4" material at will.
Rene
