Re: Covering nail holes in painted walls
Lightweight spackle or drywall compound is the right material to use. If you pushed it into the holes and wiped the surface clean before it dried, then you used it properly. If you're getting dull spots where the spackle is, that is because the wall has many layers of paint on it and the spackle doesn't, resulting in a difference of sheen. Giving the wall a second coat should blend in the spots better.
If you did not wipe the holes clean after applying the spackle, leaving a rough glob behind, then you'll have to scrap or sand those spots to blend them into the wall before repainting them. If you don't want to paint the wall twice, then dob the nail spots first. When dry, paint the wall to blend everything together.
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