I read the original question to be what does he use on the areas he has worked as he finishes them to keep them from rusting while he works in other areas. My suggestion to use ZR or some other coating other than Epoxy primer is based on the thought that he has some sort of original coating in place and that it would be too expensive to mix up a batch of epoxy for one or two pieces or areas. Once you open the catylst it has a very short shelf life so it would be a waste of $$.
I agree that Epoxy primer is the way to go when all is ready for the final bath prior to sandable primer/paint, but if the ZR is fully cured and then scuffed prior to the application of the DP primer there should be no problems from what I understand.
