There's a wall in this attic room in my house where a wall-mounted lamp and its switch were mounted on. Recently I had the lamp removed in favor of a better lighting setup and the switch relocated to a newly-installed drywall. The drywall was painted during the installation but there was further work left with the electrical so that part was left for later. As a result I'm currently left with this wall (note - it is not drywall, it's one of the faces of a reinforced concrete pillar) that has two plastered holes in it:
I have more of the exact color of paint that this wall is painted in, and want to paint the wall myself to gain some experience (it's small and its location is inconsequential). I think I can handle the painting part but I'm not sure about the prep:
The bottom hole has the opposite issue - it seems like the hole repair isn't flush with the wall; it's very, very slightly recessed from the edge of the wall surrounding it. This creates the look of a cracks around the repair since you can very slightly see (maybe the shadow of) the side of the wall around the repair.
Overall, I'm wondering how I should prepare this surface for painting. Is it enough to sand down the surfaces around the repairs with sandpaper so that they are somewhat even, and then just clean it and paint it? Do I need to apply further plaster / paste etc. to make the surface flatter, and will it work when it's already so close to flat? The hardware store said paint primer was probably not necessary since we're using the same color and the repairs are not large, is this accurate?