Aside from all the other good tips for terrain building, I will add a few that have helped me.
First, before priming the board scrap/scratch the top surface a bit so the primer has something to bit on to.
Second, since you are more then likely going to want to texture the board why not do it in one step? Buy a gallon of Drywall Paint (sometimes called PVA paint), a bag of playground sand, and a half gallon bottle of Elmers non toxic wood glue. In a seperate container mix fairly equal parts of the paint, sand, and wood glue. Stir completely adding more paint/glue or sand to get a nice thick cream of wheat consistency. Paint on and let dry. Finally paint the board whatever color you want (you can use spray paint if you want as the top of the board is now sealed and wont melt). * if you are going for a desert like board then instead of painting you can use Minwax Polyshade. This will dramatically increase the durability of the board and give a very realistic effect as seen in the link Modular Desert Board
Last tip, is be sure to cover the sides of the board with a healthy amount of Duct Tape. When you do this you want to make sure you continue the tape over the edge and around to the bottom.
Good luck.