This question already has an answer here:
Once the flooring is pulled up is it necessary to remove the dense paper layer which ultimately glued the linoleum to concrete. If it is not necessary to remove do I rough up the surface to make a more adhesive surface?
Or will thinset adhere to the dense paper layer? This paper layer has been exposed on my kitchen floor for over 3 years with spills. It does absorb but no deterioration. Help! thank you for the expert advice.