Film or oily surface after epoxy has cured

The cause is most likely due to a side chemical reaction of moisture (humidity) in the air, interacting with a curing agent (also common when the ratio's are not maintained correctly). This is known to epoxy chemists as "amine blush" or "amine bloom". Avoid conditions where moisture is coming out of the air as temperatures drop. Also can stop subsequent products from fully curing, or performing correctly and yellowing unless removed properly. To remove, thoroughly wash the cured epoxy with clean warm water, all-purpose soap and a stiff brush. Aggressive solvents like acetone will not remove the blush. Unfortunately doing resin work at night are exposing epoxies at the lowest temperatures and the highest humidity further compounding the problem.