Decorating over patched plaster

If the patches are standing proud of the surrounding wall, sanding alone is unlikely to produce a good finish. In most cases I would feather the edges using a fine surface filler such as Toupret Fine Surface Filler or Toupret Interior Filler, then sand smooth before applying a mist coat and finishing coats. Trying to sand old plaster back to level can be time-consuming and often gives poorer results than blending the transition with filler. The best approach depends on how pronounced the patches are, but from your description I would be looking at filling and feathering rather than sanding alone. Mario Decor

Sand wall down lightly with sand paper ideally using hands not a machine