The "oily" surface I put down to problems with the mixing, if the resin(the oily stuff) and the hardener are not in the right proportions then you might have resin left over.
If the hardener has dried out there might not be enough to react with all the resin.
I have had similar experiences with bought resin scenery and gluing stuff with epoxy.
It seems to seep through paint and superglue.