Water stains on an interior wall will cause black streaks and mould spots. A leaking roof or gutters could be responsible for your interior water stains. Find the cause of the water seepage and repair it before you remove the water stain marks. Mold is a bacterial fungus that produces spores that you must eliminate to prevent further staining and growth. Household bleach will remove staining and kill the mould spores. Clear the area next to the stained wall before cleaning.
Put on the rubber gloves and eye protection. Bleach is an irritant, so avoid contact with the skin.
Mix equal parts bleach with warm water in a plastic bucket. Add a few squirts of lemon juice for a natural deodoriser.
Dip the nonabrasive cloth into the bucket. Wring until damp.
Wipe the water stains on the wall with the rag until they are removed. Allow the area to dry.
Paint over the cleaned area, using primer or stain-block paint. This will prevent the stains from showing through the wall. Paint the wall with your choice of top coat when the primer or stain-block paint is dry.
Check the exterior walls for signs of water damage. You may be able to diagnose the leak source from the outside.
Keep the area well-ventilated when applying bleach and primer.