There are two reasons for power washing; the first is to simply clean and wash the outside of your home and the surrounding surfaces, such as the deck, fence, shed, walkways or driveway. Good painting companies will have a power washer that can clean any surface; the average cost for this service is between $150-$250 for all surfaces on and around the home. A standard power washing of the exterior of the home removes dirt, cobwebs, mold, mildew and any type of chalking or pollutants. Once power washed, the surface should stay clean for 1-2 years. (depending on mother nature, air borne pollutants and age of the home)
The second reason to power wash a house is to prepare it for painting. Power washing before painting usually involves using more abrasive chemicals like tri-sodium phosphate. This solution is organic so it won't hurt kids, pets, flowers or your home...but it does clean dirt, chalk and pollutants off the home. We also add a diluted bleach solution so all the mold and mildew gets killed and washed away; since it's diluted, by the time it hits the ground it's completely safe like pool water.
Power washing before painting is imperative. It's the only way to properly clean all the surfaces to ensure the new paint sticks. If you don't power wash before painting you run a very high risk of mass pealing in the near future because the new paint cannot stick to the dirty surface underneath. It's the first stage of prep work with any painting project on the exterior of the home and a minor and wise investment. All reputable painting companies and contractors will include power washing as a standard with any painting project.