RE: " flashing "
Handibond Plastic CA is specially formulated to work with "unflashed" coro. So you may not even be getting the oils out at all, its just that the glue you use sees no difference between flashed and unflashed coro.
Regular CA works better if the coro has been flashed. Other ways of flashing are out there, one method being rubbing in mineral spirits and then lightly sanding the surface to be glued.