Flaking is likely to be caused by poorly adhering, powdery old coatings. Check for the presence of powdery coating by rubbing your hand over the surface – powder on your hand indicates a problem. This may be due to any of the following factors:
- Moisture trapped beneath the surface of the paint film.
- If the paint is applied over a powdery or friable surface.
- Surface contamination such as dirt, oil, and grease.
- Polish residues on the surface, which impair the adhesion of the paint.
- Excessive movement of the substrate, such as joints, imposing stress on the paint film which can result in cracking which in turn lets in moisture, ultimately resulting in a flaking paint film.
- Resins present in knots causing light coloured paints to discolour or even flake. Resin bleed and flaking is a common problem where dark colours are used on south facing elevations. This is because dark colours absorb more heat than light colours.
Putting it right
Scrape off any loose flaking and poorly adhering coating, and wash off the powdery material with clean water. The surface should then be suitable for painting when dry. If the plaster is exposed in areas, prime these areas first and then follow with your choice of top coat.