One of the main reason we use an acrylic primer on walls is that it stops the gypsum in the plaster reacting with the cement in the adhesive (bagged adhesive) which can cause the tiles to de-bond. Is it called ettrignite Dave?
Tile adhesive bonds very to ply so no point in priming as technically wont do anything.
Cheers for the input on Prmer G Dave . Not a big Mapei user really, not that readily available to me. always love their gear though