I live 4 miles from the beach in Florida, so I have experienced what you have stated. The reason people use etch primer is to give the metal some tooth (at least that is what I have been told). Sandblasting does the same thing. So either the metal wasn't cleaned with PRE or there is something wrong with the epoxy you used. You should have no problems with rust after spraying epoxy over a sandblasted and degreased panel. At least I have had no problems.
