An MDF board measuring 210 x 297mm (A4), and 18mm deep. Please note that these measurements are from outside edge to outside edge. The frame is 8mm wide, which means the internal dimensions of the board (the space available for your mosaic) will be slightly smaller than the listed size.   Just to be clear!   The frame rises 5mm from the board surface and the depth of the entire board, including frame, is 18mm. This 5mm depth means that these boards form an ideal base for most of our glass and ceramic tiles which are usually ±3.5mm deep (but do check). MDF frames are only suitable for indoor use. 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!