Chalking is the normal aging of an exterior paint film into a loose, powdery surface. Prior to painting it is very important to remove as much of the chalk as possible. If it is not removed, the chalk will prevent the new paint film from properly adhering to the surface. Chalk is best removed by scrubbing the surface with a strong soap and water solution, or power washing followed by a thorough rising with plenty of water. After the surface is dry, check to see if any chalk still exists. If it does, another cleaning is necessary. If after the washing you have a sound paint film you can top coat with a good quality acrylic latex paint. If the condition of the existing paint film is questionable, priming is recommended before top-coating.