I believe that we need a time machine to go back and hit the first welder/boss/manager who thought it was a good idea to weld over galvanized with a brick. In the head... A few times so that this silly idea never started.
We made a bunch of duct work from 16 ga galvanized sheet. Most were at least 4 feet tall with some as big as 6ft, bent to from a 3 sided box with sheets welded on to seal it up. Lots of open corners and a few tiny fillets on some .25 thick flanges. I admit, this was a codeless venture so no testing of properties was ever done or let alone thought about....
But for what it was it serves its function well and without complaint, except from me when they bring more to weld up....
The best setup we came up with at work for welding up duct from 16 gauge was with .023 70-s6 and a 90He 7.5Ar 2.5CO2 mix blend. Its been a while and I don't remember exact settings, but im thinking about 15 to 16 volts and maybe 250 to 350 IPM wire feed? I pushed with very little to no side to side or back forth motion, or as much as the fit-up allowed. Might not work so well for a groove... maybe with motion? I did grind the galvi back from the seams but no more than .25 inch and usually not from the insides.
The He trimix ran with much less spatter and didnt burn off the galvanised as bad as say 75ar 25 co2. No idea why, but it works and it saved my arm lots of spatter burns. the .023 wire was so we didnt have to blend off a huge lumpy welds.