I am not a painting expert but I will share my experiences and what I learned doing a lot of searching. The epoxy primer will never sand well but does get a lot easier over time, like a couple of months or more. I don't know if the primer you sprayed over the epoxy was compatible or not but in either case you were likely outside the overcoat window without sanding. You should at least scuff up the epoxy before the next layer if you are outside of the time for over coating. You need to determine if the two products you are using are compatible. If there are not the same product line you may have problems.
What I have resorted to is once the epoxy primer is dry I spray 2K sanding primer over that (72 hour window for my epoxy product). Then you are able to sand easily. Even when you get into the epoxy layer a little the dust or action of the 2k primer seems to keep the epoxy from balling up.
Chuck
