Now you can apply mosaic directly, or skim coat* the board first.
Some people like to put a skim coat of cement-based adhesive over the board before they attach the mosaic to give themselves a flat surface to work on. You can also skim the back if desired and then paint it to give a neater appearance. The other alternative is to use a cement-based adhesive to affix the mosaic to the board as the depth of this will absorb the face of the washers
If the board is to be unframed, the foam edges need to be protected in some way, especially if the piece is going outside. A thin layer of cement-based
adhesive is good for this as well. It is not advisable to use grout as it has no adhesive properties and will chip off over time. The edges of the board can then be painted if required.