Spraying on an organic zinc too heavy won't get you into serious trouble, said Wood, but overly thick inorganic zinc films have a tendency to mud-crack, whether you topcoat the primer or not. Inorganic zincs are known for their porosity. A topcoat can fill up the pores and force air out, creating bubbles. The pores also can soak up contaminants such as grease, which interfere with topcoat adhesion. Organic zincs aren't porous.
Organics are much better suited to rapid topcoating than inorganics, too.
If the latter is the slightest bit undercured, the topcoat is apt to peel. Carboline's Cal Henson said that inorganics can be applied at colder temperatures than the organics in epoxy formulations. These organics normally aren't recommended for temperatures below 50 degrees, although some work at around 35 degrees. Inorganics also can stand temperatures up to 750 degrees, while the maximum temperature for organics is about 250 degrees. There have been some discouraging stories.