hi how do you know when you have grinded enough of the galavanised metal off ready for welding . Also do you need a mask when grinding galvanised metal..
You should grind it all off, you will see the colour change to a metal colour rather than the really bright Zinc colour.
If you weld over Galv, you will alomost certainly get porosity and horrible acrid smoke in your lungs on top of getting a lot of spatter. Not nice.
If its a thin layer of Zinc, (Electro plated rather than hot dip) you could get away with Mig Brazing, lower temperature, won't damage the Zinc coating if you need to maintain it for Corrosion resistance reasons.
If its hot dip, then you've no choice but to remove all of it from the area to be welded.
:S sorry, I didn't see that you was going to MMA Weld it. In anycase for a sound weld you still need to remove the Galv, or the Zinc will boil into the Weld pool and aswell as causing Porosity can also make the weld brittle.
I would def wear a Mask, especially for welding even for Grinding. Horrible stuff
You should always wear a mask when welding galvanised steel if possible do it outside and try and stay up wind of the fumes
you should also wear a mask when grinding especially with some of the cheaper grinding discs
it's much easier to weld when you grind away the galvanised and a little safer