Hi guys. Recently I have painted a ceiling of my own classroom but it seems that the paint won't stick on the ceiling because the ceiling has some white dusty particles on it. Whenever I start rolling, the first time it will stick but as I roll it down again it will come off. Do I need to scratch off the first layer first? Or what should I do? The paint I'm using is not genuine paint. I think it's something of paint mix with water that kind. Cheap one.