If levelling isn’t required, prime existing tiles with a single coat of ARDEX P 4 applied with a short pile roller or paint brush. It will dry after 30-60 minutes, producing a textured-grip surface which can then be tiled as normal using an appropriate ARDEX Cement-Based Tile Adhesive.
If levelling is required on floors, level over existing tiles with ARDITEX NA applied 3mm+ deep direct (without priming). It will dry in as little as 3 hours, and can then be tiled as normal using an appropriate ARDEX Cement-Based Tile Adhesive.