With solvent-borne inorganics, the topcoat cuts off the moisture necessary for curing. An improperly cured primer will cause the topcoat to peel right off. To determine whether an inorganic zinc primer has cured, rub it with the edge of a quarter. A properly cured film usually will shine when rubbed, while an uncured film will powder. Make the first topcoat over an inorganic zinc a mist coat.
Topcoats tend to flush the air out of porous inorganic zincs.
Escaping air can make the topcoat bubble. The mist coat fills up the pores. Then apply a normal dose of the topcoat. Observe proper spraying techniques with whatever primer you're using. Holding your gun too far away from the surface gives the solvent more time to evaporte before it hits its target, leading to dry spray.