Probably the Styrene in the Body Filler being drawn out by the application of the Epoxy. Most Body Fillers need sealing especially if "freshly" applied and still green.

Carbon Copies Ltd

Just found out that there are apparently some incompatabilities with epoxy and polyester resin and that one cannot be laid over the other and visa versa.  Is that true?
I made a part with epoxy and then used some polyester body filler on it before putting some more epoxy in it which has not cured properly, I was wondering, could the body filler have affected the epoxy?