Exterior 12mm Treated Plywood Numbers.
 
Base measures 250mm.
 
We highly recommend that before starting your mosaic numbers you seal the board.  A mix of 1 part PVA to 2 parts water can be used and brush both sides and along the edges with the mix and allow to dry.
 
After grouting your mosaic we recommend painting the back and the sides of the base.  You also can seal the mosaic with a grout sealer.