Hi Mark31,
As everyone knows, welds over galvanized surfaces have a tendency for porosity. However, small cracks may develop in fillet welds. The porosity may be within acceptable limits, but the cracking may pose a difficult problem. Investigations have revealed that weld metal cracks in galvanized steel are intercrystalline with a zinc rich layer lining the surface of the crack. On a galvanized surface, the coating is not usually uniform, and the cracking severity will vary with the varying coating thicknesses. Cracking can also be caused by restraint, especially in fillet welds, and the tendency for cracking will increase as the base metal thickness increases. To reduce restraint, the joint can be fitted with a small gap, which also allows volatile zinc to escape.
I agree with John's tip about oscillation, etc., and will add that the weld will contain porosity and possibly cracks if the zinc is not completely volatilized in the area where the weld metal is to be deposited. Also, on base metals 1/4" or thicker, insufficient penetration may be a problem with fillet welds. Always use the largest diameter electrode compatible with the base metal thickness and welding position, and with a whipping motion, concentrate on volatilizing the zinc ahead of the arc.
6010 and 6011 electrodes have been recommended for this, and as John said are sometimes used, but since most job specifications require 70 series electrodes, make sure that the job specifications allow for the use of 60 series.