I have run into that problem before and the epoxy will peel off like skin on a banana. I'm pretty sure it's a problem of surface tension from spray paint like the metallics that give the epoxy NO bite to grab ahold of. There is some compatibility problem so a simple clearcoat sprayed on to give you something to scuff with a scotchbrite pad will help give the epoxy something to take hold of. That may be why the one you used some clear on went well.
good luck
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