What Does Chalking Mean?
Chalking is a powdery, friable layer on the surface of a coating. It is normally caused by exposure to UV light or other forms of radiation, like nuclear.
Chalking is especially prevalent with flat paints and white or very light-colored paints that contain high levels of titanium dioxide and extenders. A low degree of chalking is often beneficial to whites and off-whites, since it tends to rid the surface of a certain amount of dirt and mold.
Excessive chalking is detrimental because it can:
- Lighten the color of the paint
- Erode the paint film, resulting in a loss of protection
- Run down onto the underlying structure and deface the appearance of the surface
Chalk must be removed before repainting and can be considered similar to dust and dirt.
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Corrosionpedia Explains Chalking
Chalking often results from insufficient resistance of the coating to ultraviolet (UV) light.
Causes of chalking include:
- Using a low-grade paint
- Over-thinning the paint
- Overspreading the paint
- Not priming and sealing a porous surface
- Extended exposure to the combined effects of moisture and the ultraviolet rays of the Sun
UV light or radiation breaks down the bond between molecules in the coating film. It is most common in epoxy coatings, but can be seen in almost all coatings left exposed to causative conditions for a long enough period of time.
Old paint is prone to chalking. It is to be expected and is worse if enamel paints or lower-gloss-level acrylics have been used.
Proper surface preparation is required before painting to prevent chalking. The remedy is to utilize a topcoat with greater resistance to UV radiation. Quality paints may chalk mildly, but still keep a sound surface that does not crack and retains good moisture and weather resistance for many years.
Chalking can be treated by:
- Rubbing the surface with a finger or dark cloth to determine the degree of chalking
- Removing all chalk residues
- Allowing the surface to dry thoroughly
- Rubbing the surface with a finger to check if any chalk residue remains
Severe chalking may require pressure washing or sandblasting. Moderate and lightly chalked masonry surfaces may require wire brushing or sanding to remove the excess surface powder.