One viewer installed drywall, taped the joints, painted and then discovered that there were lumps and uneven areas in the joint plaster. What to do?
It is certainly easier to perfect the sanding before there is paint on the wall, so the moral is to really verify your sanding job before saying it is good enough. Things that barely show up in the white plaster can look like a disaster under a painted surface. Run your hand over all the joint compound to feel any uneven spots and look carefully at the wall with good lighting and your eye looking down the wall at an angle to spot those imperfections.
After the paint is on you have two choices. You could sand through the paint and then level the joint compound, or you could plaster over the paint. The latter is much easier. Just add more compound, perhaps even two coats until you do have a perfectly smooth wall. With either technique you must put a primer/sealer over the bare compound before painting or the paint will come out lighter over the new plaster than on the rest of the wall.
To see a video on all of this check out Taping Joints in the Learning Curve tab: Drywall.