Variations in temperature of the inflexible render causes cracking. Cement render, no matter how it is formulated (usually a ratio including sand, lime, cement and water) is highly alkaline and extremely rigid. After it is mixed and applied to the building it has to be left for 28 days to cure which allows the alkaline to stabilise before it is painted. After being painted, the render is usually exposed to extreme variations in temperature which causes it to expand and contract. This ongoing stress on an inflexible product causes the render to crack. These cracks appear first as hairline cracks and are more obvious when the render is wet.