After checking out this thread, I was curious how you would treat white board paint. Recently, in our conference room we painted a wall with the white board paint. We had to do 4 layers and counting! The marker imprints still linger and it looks like we're going to have to go for a 5th layer. We didn't do any sanding, but I assume that isn't necessary for this application? What brand(s) are the best for it? Is this normal?