Sandpaper is marked with a number on the back according to the coarseness of the grit, with a higher number denoting a finer grit. Usually, you'd want to sand off the enamel paint to paint with latex or another water-based paint over it. If this is the case, all you need to do is to dull the shine of the enamel paint; you don't need to sand off every speck of it. If you use 320-grit sandpaper and you're not making any headway getting off the enamel paint, switch to a rougher paper such as 200-grit. Make sure you're not leaving scratches and grooves in the wall. Although you can use an electric power sander for the wide, flat areas of the wall, when you get closer to the corners, floor and ceiling, you may have to sand by hand.