A black or green growth on your painted walls is quite possibly mildew. Mildew is not as harmful as mould but it will still stain the surface it is growing on as well as fill the area with an unpleasant, musty odour. Simply painting over the mildew will not get rid of it and the mildew will eventually make its way back to the surface. Fortunately, you can remove mildew from the painted walls with a cleaning solution made from a few inexpensive items.
Provide better air ventilation and circulation during the cleaning process. Open a window, door or turn on a fan. Wear a pair of rubber gloves.
Fill a spray bottle with 480 ml (2 cups) of cool water and add one drop of liquid dish soap. Swirl the bottle gently to mix the contents together. Mist the mildew with the mixture and immediately wipe the wall clean with a cloth.
Combine 4.5 litres (1 gallon) of water with 120 ml (1/2 cup) of oxygen bleach. Oxygen bleach is a safer alternative to chlorine bleach. The oxygen bleach will not discolour or damage surfaces yet it is still an effective mould and mildew killer.
Submerge a sponge in the bucket and wring out the excess liquid so the sponge is damp but not dripping wet. Gently scrub the mildewed area of the painted wall with the sponge. Let the mixture sit on the wall for 10 minutes.
Dampen a clean cloth in cool water. Rinse the mixture off the wall with the cloth. Use a towel to remove excess liquid off the wall and point a fan toward the wall to dry it.
Test the cleaning solution on an inconspicuous area of the painted walls. If damage or discolouration occurs, discontinue use.