hi, how do you remove emulsion paint from a plastered wall?

[Keith J]

have removed the old wallpaper from the walls and some of the emulsion has come off with it, l now need to remove the rest of the old emulsion down to the plaster because we want to paint the wall and dont want jaggered edges where some of the old paint has stayed on. thanks
have tried sanding before till its really smooth but it still leaves jaggered edges, have thought about using a steamer but not sure if this would 'blow' the plaster