There is only one way to deal with welding zinc coated steel from a quality, safety and economical stand point. Remove it ----- grinding or, if approved, burn it off, eliminate it from the weld zone. It can be done with E7018 ( preferred ) or E7015, E7013 ( from years ago, no longer available I believe ) or E6011. BUT the main thing here is the skill and talent of the opertator must be perfect. The zinc at arc temperatures will mostly turn to a zinc gas and at the density of the weld puddle ( all things considered ) WILL get trapped in the weld as "porosity" to various degree, again depending on the "skill and talent" of the operator. How much porosity is acceptable, is the welding going to be on heat sensitive steel, verticle up and over head (the puddle will be difficult to control and bead cross section will be hard to maintain) ????? If it is on a structural ar bridge porject, an experienced welding inspector will zero in on those welds real quick and repairs will be VERY costly and very aggravating.