An MDF board measuring 50 x 50mm. These boards are 12mm thick. Only for indoor use as these will not enjoy getting wet. It is good practice to prime your board before use. Paint the surface with a quick coating of 50:50 PVA glue and water; this acts as a sealant and stops water from your tile adhesive penetrating the board. If you intend to put hanging hardware on the back of your board, it’s worth drilling holes before you start work - if you do it once the tiles have been stuck down you may loosen them. But don’t worry, they can always be stuck back on!