By far the largest problem you will incur will be that you will not be able to get rid of the galvanized coating at the weld interface. When you projection weld you will need to keep watch on a few key things.
1. Make absolutely sure that you have very good followup. This means that when the projection collapses, the weld head must come down equally as fast. If the head is very heavy, it will not follow up very well. There are low inertia heads that use specially sized springs to ensure fast follow up. This would be a good way to go.
2. It would be best to try to make the weld with high currents for a short weld time. This in effect blows the galvanized coating away from the weld interface so that there is a clean area for the projection to weld to. Going to long weld times will not help the situation. (this is a problem that many people have when trying to projection weld on galvainzed material.
