If dirt, particles of dust and debris (or any one of the three) accumulate on the paint film, it can look like mildew.
Painting with a low-quality product, especially when it is semi gloss or satin paint.
Soil from the ground splashing against the siding.
Air pollutants such as car exhaust and flying dust particles collect on the sides of the house and the horizontal trim.
Washing the surface prior to priming and painting.
Use a detergent solution and a scrub brush.
Rinse the area thoroughly using a garden hose.
You might need to use a power washer when heavier accumulations of dirt are present. It is not possible to eliminate all dirt accumulation. However, latex exterior paint of the highest quality resists dirt pickup and is easy to wash.
Paints with a higher gloss are more resistant to dirt pickup than flat paints. The reason is that flat paint is more porous, which enables it to easily entrap dirt particles.