Welding on galvanized steel can be dangerous - it gives off very hazardous zinc fumes. If you have to weld galvanized I would strongly suggest grinding the zinc galv coating off in the area to be welded - and use real good ventilation. Why not just put the pipe posts in at the standard guard rail hole spacing (6'-3" I believe) and mark and either drill through the pipe or blow a hole thru the pipe or weld a non-plated 3/4" hex bolt head to the pipe then mount the guard rail with a washer and nut... Good luck - watch those galvanized fumes! Jim at Dawn | 
Best way to attach guard rail to pipe?
