Blackboard paint

So I bought blackboard paint and painted a big piece of one of the walls in our living room with it. When it was all dried and finished I explained to the boys that they could now write on this wall with their chalk. They were very happy with my surprise.
‘Wow,’ said the first. And he started scribbling on the wall.
‘Wow,’ said the second. And he walked to another wall and before I realised what happened he had scribbled some huge scratches on our normal walls.
Aaargh!