[Forrest]

Not sure what that epoxy you used but it should not be easily sand-able. Sounds like yours is hardening way too much( I use one coat of shop line jp375) and it will "ball up" when you try to sand it because it is more flexible. Also 3 coats seems alot..should use very minimum (1 coat). Then I use a thin film of sandable polyester primer.( I use featherfill g2.) And then block till you just see the epoxy primer underneath.. The whole point is MINIMUM coverage/thickness of the primers and final paints.. Of course all this works for me MOST of the time but painting is unpredictable at times!