What may be happening is...... you're putting the top coat on too soon. You should let the epoxy set for at least an hour before top coating. The top of the epoxy is air drying, the middle of the epoxy is not curing. So when the paint chips, the top surface of the air dried epoxy is sticking to the top coat, and the bottom part of the epoxy is sticking to the metal.
The first coat of epoxy is not sticking to the second coat of epoxy.