Not a straight forward answer. I have got a couple of these going on at the moment. I usually recommnd the turbo disc from SPE Ltd which fits most screeds to produce a key. Even that has failed to cut through the two in question. I guess the only sensible answer is treat it as a non porous surface like a powerfloated concrete in terms of primer. Or use a primerless system. As unprofessional As it might sound I have found pre mixed tubbed adhesives really useful in this sort of situation but that is not to proper tilers tastes generally especially on a big floor. It' about getting the primer to wet the surface of the screed and not just sit on top. If the primer soaks in then a gypsum based adhesive will be fine. If the primer sits on top then use a different primer. Epoxy can be useful here.
