After the stains disappear, the surface should be thoroughly washed with warm, fresh water to remove the oxalic acid and any traces of the chemical causing the stain. If all sources of iron are not removed or protected from corrosion, the staining problem may reoccur. Caution should be exercised when using oxalic acid since this chemical is toxic. Chalking results when a paint film gradually weathers or deteriorates, releasing the individual particles of pigment. These individual particles act like a fine powder on the paint surface.
Most paints chalk to some extent.
This phenomenon is desirable since it allows the paint surface to be self-cleaning. Some paints or stains chalk badly and can discolor a lower surface as they wash down over it. Discoloration problems from chalking can be prevented by selection of a paint with the appropriate chalking tendencies. The manner in which a paint is formulated may determine how fast it chalks. Therefore, if chalking is likely to be a problem, select a paint which the manufacturer has indicated will chalk slowly.