Before sticking an uncoupling mat to an anhydrite (or gypsum or calcium sulphate) screed, you must first ensure it is dry and suitably prepared; please refer to our How do I tile to anhydrite screeds? FAQ for further guidance. In the case of both anhydrite screeds and gypsum floorboards, they must then be primed with a single coat of ARDEX P 51diluted 1:3 with water, applied in one continuous coat using a brush or broom. Once the ARDEX P 51is dry, the uncoupling mat should then be fixed using ARDEX AF 200 PLUS or the appropriate ARDEX Cement-Based Tile Adhesive.