Before tiling to plaster, you must first ensure it is at least 4 weeks old and that the total weight of the tiling will not exceed 20kg/m2, as recommended by BS 5385. You must then wire brush the surface to remove any dust. The plaster should then be primed with a single coat of ARDEX P 51diluted 1:3 with water and applied with a brush. Once the ARDEX P 51 is dry, tile as normal using an appropriate ARDEX Cement-Based Tile Adhesive.