My first coat after a lightly sanded XIM primer is opaque white on all plugs, heat set, then colors. I use Createx and Golden. Does this happen after you epoxy the plugs and then hit a rock and compromise the epoxy and then starts peeling ?
If it peels right off the primer I'd try some different primer then. The problem seems to be adhesion to the primer.
Maybe give XIM 400 white a try. Not cheap but I like it a lot. I get it here in the spray can.
http://www.thepaintstore.com/XIM_400_White_p/1102.htm