Power washers, also called pressure washers, are effective tools for cleaning the outside of a house. They also work well to remove flaking and peeling paint from wooden siding before you repaint. However, it's important to get the right size machine for the job. A power washer that provides too little pressure won't remove all the paint. One that sprays with too much power will take off the paint, and also damage the wood underneath.
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Use pressure washers from 1,200 psi to around 2,000 psi to clean surfaces before painting but don't expect them to remove much paint. Stronger machines, starting at about 2,500 psi, work better to take off flaking or peeling paint. For more stubborn jobs, consider a 3,000 psi pressure washer, which can exert the force needed to break paint off the surface of the wood.
Start by covering all outdoor lighting and electrical fixtures with plastic bags, closing the doors and windows and covering landscaping plants and lawn. Wear goggles and protective clothing to prevent damage from pressurized spray. Hold the nozzle between four and 12 inches from the painted surface, at about a 45 degree angle, to remove paint from wood. Avoid spraying under the laps of siding, or into vents, since this can cause moisture problems later.
According to Sutherlands paint company, very high powered pressure washers can damage wood or other paint substrate. Avoid using a pressure washer over 3,000 psi. Instead, work slowly and carefully to remove all flaking paint. Paint that won't come off from 3,000 psi or less can be hand scraped or simply painted over, as it is unlikely to peel.
Using a pressure washer makes things easier, but it won't keep you from having to hand scrape some parts of the house. Water blasting removes most of the peeling or blistered paint, but also tends to make the edges of remaining paint areas curl up. Allow the siding to dry, then use a scraper on these areas before painting to ensure a flat surface.