Walls need to be prepared for painting by cleaning and sanding.
It takes little time for the walls in your home to accumulate a thin film of grime, grease, and dust. And walls often look clean when they are not. Dirt and grime between the wall and the new paint must come off, because they will interfere with the adhesion of your new coating. If you think your walls are clean, dampen a paper towel with a little vinegar and gently rub a small place on the wall. You're sure to see the evidence on the paper towel that doesn't show up noticeably on the wall.
Phosphate residues weaken paint bonds, so use a low-phosphate household cleaner or brew your own with 1/4 cup of trisodium phosphate (TSP) in a gallon of water. Use the procedures discussed in "Good cleaning techniques," below.
Sanding walls provides a uniformly smooth surface with a slight "tooth," which helps the paint stick more securely. If you scrub flat or semigloss paints with a scrubbing strip, you have effectively wet-sanded them and won't need to go further. Glossy surfaces will need sanding with a 100-grit sanding screen.
Some walls need to be sanded, others don't. Flat or semigloss surfaces cleaned with a low-phosphate cleaner and scrubbing strip won't need sanding. Glossy paints will -- sand them with a sanding screen and wipe down the dust. In all cases, fix cracks and patch holes before sanding.
Mask off the baseboard and the floor area below the mildewed surface. Sponge on a 1-to-3 solution of bleach and water and scrub the mildew with a sponge. Let the solution set for 20 minutes to kill the mildew, keeping it wet during this time. Rinse the area thoroughly and let it dry overnight.