Prepping a plaster wall for painting
Sanding and spackling?


You could probably make it look OK with spackling followed by sanding.

Better results could probably be had by using joint compound followed by sanding, using progressively larger drywall knives. This method (which is the same used for joints in new plaster-board/drywall application) will result in a wider repair band, feather sanded smooth, which should be less noticeable.

Either way you will be left with an area that has a different texture than the rest of the wall. Depending on your need for perfection this can be remedied to some degree by using a thick nap paint roller and painting the whole wall, or even texturing the whole wall. Visibility of texture differences can be affected by things (oblique lighting and glossy paint tend to make it more visible).