When coating an interior wall, paint is usually used, applied with either a brush or a roller.
Exterior walls, on the other hand, require a more substantial covering than just paint, to give maximum protection against the weather. However, like painting, exterior walls often need several layers, and each should be dry before the next is applied.
Because exterior walls usually require thicker, more protective layers, tools such as air or manual sprayers are used to get this coating on quickly and easily.
Unlike the smooth action of a roller or brush, a coating sprayer purposefully creates a rough, uneven texture on exterior walls. This is to ensure that rainwater drips off the protruding parts of the wall surface, rather than running down the wall, saturating it.
This keeps the wall dryer and less susceptible to damp.