we had trouble with even a flexible poly glaze filler sticking to it. We stripped it back off in sections and covered nearly the whole thing with bumper repair. The flexible filler then stuck pretty well on top. In the few areas where the flexible filler was adhered to the endura rubber we sprayed SEM adhesion promotor, and not sure if it's solvent base. So as far as materials used we were in the dark. I gathered what info I could find on-line but it was nothing compared to the insight Barry is letting us in on right now. Wish I read this post BEFORE we fixed it, and it makes total sense about creating a barrier coat. I just didn't have a good feeling about ANY polyester flexible filler adhering to the endura rubber no matter how good it feathered out after adhesion promoter was applied. I just didn't know better at the time.
Anyhow, I did make a video on the repair and if anything it can be helpful in addressing fitment issues, as it seemed to be a headache making it fit right.