best flux core wire to use

I have the same HF $90 welder. Don't be dissuaded. Many here will say only buy a >$500 Lincoln welder, but my Dart cost $1000. That HF welder will weld fairly thick steel. I fixed cracks in the ~1/4" thick AC compressor bracket on my Dart. My greatest success was welding a steel strap to fix a cracked LCA pivot on my 84 M-B that salvaged the car. My son has been driving it for 2 years since. I butt-welded a floor pan into my Dart, but I would lap-weld it if I repeated that job. I am not too worried about the welds cracking, but it was very tedious to cut the sheets to sit flush and I had many burn-thrus. A lap-weld is easier to form and stronger. One guy here said he runs his HF welder from a 20A dimmer switch for sheet-metal and it works great at avoiding burn-thrus. I bought one to try next time. For outside sheet metal repairs, I would think butt-weld is best so you don't need a lot of body filler. I'll be getting to that soon on my Dart. I am still on my original flux wire spool from HF.

My greatest difficulty is seeing well as I weld. I don't know if it is too much out-gassing or the auto-adjust helmet from HF is too dark. Maybe both. I often end up laying a bead in the wrong spot.