The substrate is the basis for the tile assembly. Professional tile installers will tell you that a tile installation is only as good as what it is bonded to. So, before planning a tile project, it is important to determine if your substrate is acceptable to receive an interior tile installation.
If your substrate is not structurally sound, solid and stable, it is recommended that you replace it with an appropriate material.
All interior substrates should be clean, dry and free of dust, dirt, oil, grease, paint, tar, wax, soap, concrete curing compounds, concrete color stains, concrete sealers, clear coats, loosely bonded toppings, any primers that are not MAPEI brand, old adhesive residues (unless otherwise recommended by MAPEI), and any other substance that may prevent, reduce or affect adhesion or performance.