I get two calls related to this about once a month.
1) The edge of my frame is showing flash rust through the epoxy.
2) no adheision in the edge of this panel OR Flash rust is coming through epoxy on edge of truck quarter by the bed.
All related and easy to miss, make sure all edges get a wet coat and on a frame what I like to do is spray two coats and after second coat, with no waiting I will do a third coat just waiting the edges.
Truck bed quarters , its very easy to have the edge on top covered with over spray because of the difficulty to spray on some of these trucks.