The Issue
Sometimes a chalky white layer can appear on exterior walls. Whilst some exterior paints are designed to do just this, sometimes it can be the sign of an issue.
The Cause
There are a number of reasons Chalking can occur:
Water borne paints being applied in temperatures below 5 degrees.
An interior paint being used for an outdoor application
Inadequate sealing of a porous surface
Over-thinning a coating
Some exterior paints are designed to chalk as part of a self-cleaning mechanism, so make sure to check the product data sheet first.
The Solution
Remove chalk by washing or power washing the surface with a surface appropriate cleaner.
For best results firstly ensure that the correct primer/base coat is being applied. Applying a Stabilising Solution or an Alkali Resisting Sealer before coating will help to reduce the risk of chalking.
Secondly, always check a products data sheet and back of pack to double check that it's the best product for your chosen surface.