Mildew grows in warm, damp conditions and so may develop on either interior or exterior plaster walls. The same products will be used to clean plaster walls, whether inside or outside the home.
Put on rubber or plastic gloves and eye goggles. Wear old clothing. Open all windows and place a fan in the room and turn it on.
Pour 1 gallon of water in a bucket. Add 3/4 cup of bleach. Stir the solution thoroughly with a stick.
Dip a sponge in the bleach solution. Scrub the mildewed plaster, using firm strokes. Continue until all the mildew is removed.
Wash the bleach solution off the plaster using a water-dampened sponge or rag.
Leave the windows open and the fan running until the plaster has dried.
Spray the plaster with water from a hose to clean off any dirt and debris.
Wear eye goggles and old clothing. Add 3/4 cup of bleach to 1 gallon of water in a garden sprayer.
Spray the mixture on the walls, avoiding any metal or wooden trim. If any trim is inadvertently sprayed, wipe it immediately with a clean, damp sponge or rag.
Allow the mixture to remain on the walls for at least 20 minutes.
Rinse the walls with the hose, starting from the bottom of the wall and moving up. Allow to dry.
Flush the garden sprayer well with clean water after use.
Cleaning with bleach stains many paints. If the plaster is painted, you will need to apply new paint after cleaning the plaster.