I firstly tried f/core, but a lot of burn through and very difficult to make a nice weld, which will heat up the panel too much trying to fill in the burn through. It was impossible not burning through at times. Use gas as the other members stated, and use 75/25 mix. You will be so surprised at the different results. I know i was.
Make sure you do some practise on scrap panels. I wanted to do butt welds on my panels, so did lots of practise time on scrap.
I have Hobart HH187 and had mine set at 2 clicks on voltage and 35 wire speed and gas at about 20-25.
If you don't have enough gas running to the mig, you will get welds that look like little craters when butt welding, that will tell you to turn it up. But this is not to say that i am spot on with the settings.
The experts here will chime in correct me where they see i am wrong.
The only way to know is to try it yourself and practise, but get the gas, you won't be sorry.