Epoxy primer is a good type of primer paint to use on older metal as well as newer metal. Before applying a new coat of epoxy primer or the color coats, the underlying primer coat must be fully dry. On a hot and dry day, or if you’re lucky enough to have an automotive oven, this can take as little as thirty minutes. But on a cold or humid day, it can take up to two or three hours. If the weather is both cold and humid, you may actually need to wait overnight for the primer to dry completely.
For more info, see Self Etching Primer vs Epoxy Primers