There are two main causes for flaking – one could be overpainting grease, dirt or other contaminant or applying paint to a porous or powdery surface. It can also happen if you use a water-borne masonry paint over a solvent-borne paint. To overcome these problems with adhesion scrape away all the loose paint to a firm edge and abrade any glossy surfaces. For solvent-borne products it may be necessary to recoat in with another solvent based coating – for example Sandtex Trade 365 All Weather Masonry.