I'm finding better results leaving a wider gap between the two pieces you are welding together and the wider gap allows you get the weld hotter and stay on it a bit longer and get a better penetrated weld. Then when you grind it you have more weld to grind, which is good.
The little rectangle on my firewall, i fitted the piece as a perfect fit with minimal gap and then I couldn't stay on it at all and when I grinded it down I didn't like the shallow weld so i am going back and making a gap with a cut off wheel and going over it again.
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