What can i use to install tile on top of old tile ?

Thank you for your inquiry regarding Mapei products! In order to tile over tile, you must do one of the following two procedures. Process #1, Grind or sand down the top layer of your existing floor tile installation. Prime the installation using Mapei Primer T, diluted 2 parts water to one part Primer T. Roll on primer for 100% coverage, let dry and install your new tiles over the old ones using the Large Format Floor and Wall Tile Mortar. Process # 2, Make certain that your existing tile installation is clean and dry and sound. Roll on 2 coast of the Mapei ECO PRIM GRIP. First coat rolls on north and south, let dry, then roll on the second coat east and west. Once dry (approx. 2-3 hours), tile over this primed floor using the Mapei Large Format Floor Tile Mortar, Large Format Floor and Wall Tile Mortar or Large Tile & Stone Mortar. Please call us directly at Tech Support for any further discussion you wish to have regarding your project. Tech Support can be reached Mon-Fri from 8 am to 5:30 pm EST at 800-992-6273. Thank you! Product Advisor # 11