Last night, The Wife grew bored with the drawing of the Colosseum on the blackboard and erased it. Or tried to, anyway. The chalk smeared around but stayed on the board, and so we concluded that we had built a blackboard that we could write on once. We had been hoping for a the kind that we could both write on and then erase, but we apparently hadn't done that.
A close reading of the instructions on the chalkboard paint indicated that the manufacturer had intended that it be used on a non-porous surface. That turns out to not be plywood, no matter how well-sanded. So my interim solution was to re-blacken the surface of the board and then cover it with a non-permeable substance. Enamel paint and polyurethane are the tools I have; the board is even now taped up and on the work table in the garage for the first coat of black enamel to finish drying. Tomorrow is for the polyuerethane.