The upper walls in my bathroom are painted with bathroom paint which we intended to paint over. However, as the walls were a bit rough I decided to sand them down first which has led to the paint flaking off big time. It looks like there was just one coat of paint immediately over the plaster. Do I need to remove ALL the paint before I can repaint and if so, is there an easier way than sanding. Also, do I need to do anything to the plaster before putting on new bathroom paint. I am a total novice at all this and any help would be appreciated.