You can weld over gal on mild steel and it is not likely to break but, as has already been said, the fumes are toxic and it will spit.
I was working for a large structural steel fabrication company in Wagga Wagga around 1980 when the company got the contract to build 2000 galvanised gates and many hundreds of fence panels for the new cattle sale yards at Bowen just north of the city. Two of us started by building an elaborate jig for the gates. I welded one side then flipped the gate over and slid it down a short ramp so the other man could weld the other side. I calculated we each did around 75,000 welds. All of the welding was done with 320 amp MIGs and CO2 gas which makes the spitting even worse.
We would still be building them if we ground the gal off first. To protect ourselves we wore twin filter respirators and used a big industrial fan positioned behind us so we could use our bodies to shield the weld gas from the fast moving air. I always held my breath during each weld then moved slightly allowing the fan to clear the fumes instantly.
It worked, we both survived and we consistently made 60 gates a day.
For a little one off job like yours, I would simply grind the gal off. I have done this many times at home on small projects and it usually only takes a few seconds.