Every spring in Atlanta, a yellow film of pollen settles onto every exterior surface, and every summer, humidity grows mildew on the shaded sides of the house. Add the chalk that aging paint sheds and you have a layer of grime sitting between your siding and any new coat of paint. Paint applied over it bonds to the grime, not the house, and peels early.
Pressure washing removes that layer, but the pressure has to match the material. Siding and painted surfaces call for a soft-wash approach: lower pressure, a cleaning solution that kills mildew, and a thorough rinse. Concrete driveways and walkways can take much higher pressure. Wood decks sit in between, and too much pressure will scar the grain permanently.
The dry-out afterward matters as much as the wash. Wood siding and trim need at least a day or two of dry weather before paint or stain goes on, or moisture gets trapped under the finish and pushes it off from behind. A good painter builds this into the schedule rather than washing in the morning and painting in the afternoon.
Pressure washing is also worth doing on its own. If your paint is still sound but the house looks tired, a wash often brings back the color and buys you a few more years before a full repaint, at a small fraction of the cost.