There are conflicting messages about using pressure washers when prepping building exteriors for painting. Some insist that pressure washers should never be used while others maintain that they are just fine and use them both for paint removal and cleaning. Which is accurate? As with most things, the truth is somewhere in between and depends on circumstance.
First, it is true that pressure washers should never be used to REMOVE paint. In order to remove paint effectively you need to dial the water pressure up to blast away the old paint layers. Unfortunately, this also means that the wood siding or stucco can be damaged or eroded. This is especially true on historic buildings because they typically have areas which are in good and others in poor condition (often on the same piece of siding). Remember, pressure washers are powerful enough to mar stone such as granite, so even at a relatively low setting a wood substrate can be affected. Some would argue that they can dial the pressure down so that it won’t damage the wood. However, in order to to make a pressure washer effective, the pressure has to be set fairly high. The photo below shows clapboards where wood fibers have actually been torn away from the surface. The best and safest way to remove paint from historic exteriors is by scraping and sanding because it allows you to vary pressure according to the condition of the siding.
Another problem with pressure washers is their tendency to drive water into cracks and gaps and saturate the siding itself or the sheathing behind it with water. When water has been driven behind the siding it can take weeks for it to dry. As the water evaporates and escapes through the siding as a vapor it can cause paint failure. Even the best primers and paints will not adhere to damp wood or wood releasing water vapor. The photo below shows siding on the same building that has more eroded wood as well as cracks around nails which would allow easy access to water from a pressure washer.
Does this mean pressure washers should never be used? No, not really. They can be fine when used at their lowest pressure setting as a means to CLEAN a wood or stucco surface. Care must be taken to direct the water stream at a correct angle so that water won’t be driven underneath clapboards, battens or molding. However, a garden hose with a brush is just as effective and far less likely to damage the wood siding or drive moisture behind the surface. As is often the case when working on historic buildings, shortcuts can lead to poor results and there usually is no substitute for a little elbow grease.