A pressure washer can strip loose paint, dissolve years of chalked pigment, and flush mildew out of lap joints in a fraction of the time any hand-washing method requires. It can also split wood fibers, force water behind siding panels, and leave a surface so saturated that paint applied two days later still fails to bond. The difference between those two outcomes comes down to selecting the right pressure settings, using the correct nozzle, and following a drying protocol before paint ever touches the wall.

Power washing is not optional prep. It is the single step that most affects how long an exterior paint job holds. Dirt, chalk, and biological growth sitting under new paint guarantee early failure. A clean, dry substrate is what makes primer and topcoat stick for years rather than months.

Why Power Washing Is Essential Before Exterior Paint

Every exterior surface that has been exposed to weather for more than a year accumulates a layer of contamination that prevents paint adhesion. Chalking is the most common issue on previously painted surfaces. As UV radiation breaks down acrylic or alkyd binders, the surface layer converts to a fine powder. Running a hand across an aging painted surface and seeing white residue on the fingers is evidence of active chalking. Painting over chalk produces a new coat that bonds to the loose powder rather than to the surface itself. That coat peels within months.

Beyond chalk, exterior siding collects airborne dirt, pollen, algae residue, spider webs, bird droppings, and oxidized pigment. Each of these contamination layers acts as a barrier between the substrate and the paint film. A power washer removes them in a single pass.

For surfaces with mildew, washing is even more critical. Painting over active mildew does not kill the spores. The organisms continue to grow beneath the new paint film and eventually push through it. The only way to stop this cycle is to kill the mildew chemically during washing, before any new paint is applied.

There is also a mechanical benefit to power washing on wood siding. The high-pressure water raises the grain of weathered wood, creating a slightly roughened texture that improves mechanical bonding between the wood and the primer coat applied afterward.

Correct PSI Settings for Different Siding Materials

Not all siding tolerates the same pressure. Using too much PSI on the wrong surface causes damage that makes painting harder, not easier.

Wood siding: Stay between 1,200 and 1,500 PSI. Soft woods like pine and cedar are susceptible to fiber damage above this range. A 25-degree green nozzle tip at these pressures cleans effectively without raising excessive wood grain or forcing water into lap joints. Lap siding should be washed from above at a slight downward angle so water does not drive behind the boards.

Vinyl siding: Vinyl handles 2,000 to 2,500 PSI without damage. This range removes oxidation buildup and the chalky film that forms on older vinyl. A 25-degree tip remains the right choice for general cleaning; switch to a 40-degree white tip near edges and trim to avoid lifting panels.

Brick and masonry: Brick tolerates higher pressure, but mortar joints on older homes can be eroded by aggressive settings. Keep brick between 1,500 and 2,000 PSI and hold the wand at least 12 inches from the surface.

Stucco: Stucco is the most pressure-sensitive exterior material. Use 1,200 to 1,500 PSI and a 40-degree tip. Hairline cracks in stucco widen quickly under high-pressure water, and once water gets behind stucco, the surface prep problem becomes structural.

For the equipment, the Sun Joe SPX3000 at 2,030 PSI with a 34-inch extension wand covers most residential siding cleaning tasks well. Its dual detergent tanks allow a cleaning solution to be applied on a first pass and rinsed on the second without stopping to switch containers. For larger homes or heavily soiled surfaces, the Sun Joe SPX4600 at 2,300 PSI provides additional power without requiring a gas unit.

Keep the wand moving continuously. Pausing over one spot concentrates the full pressure on that point and causes streaking on paint or etching on softer materials.

How Long to Wait After Power Washing Before Painting

Wood needs 24 to 48 hours of drying time after pressure washing before primer or paint is applied. The waiting period is not arbitrary. Wood absorbs water into its grain structure during washing, and that moisture must fully evaporate before paint traps it inside the surface. Painting over wet wood is one of the most reliable ways to cause blistering within the first season.

The 24 to 48 hour range assumes dry weather conditions with temperatures above 50 degrees Fahrenheit and reasonable air movement. In humid climates or overcast conditions with low airflow, extend the wait. A moisture meter takes the guesswork out of this decision entirely. Probe the wood siding in multiple locations and confirm the moisture content is below 15 percent before priming. Readings above 15 percent indicate the wood still holds too much moisture for reliable paint adhesion.

Vinyl, aluminum, and masonry dry much faster than wood. These surfaces can often be painted 12 to 24 hours after washing in normal conditions. Even so, they should be visibly dry and free of standing water before any paint application begins.

Avoid washing on a Friday afternoon with plans to paint on Saturday morning. That schedule does not leave enough drying time in most conditions.

Areas to Avoid Power Washing and How to Clean Them Instead

Direct pressure washing is inappropriate in several locations around a house exterior, even when overall PSI settings are correct for the siding type.

Electrical components: Outdoor outlets, light fixtures, and junction boxes should never be hit with a direct stream of water. Mask these before washing or clean them by hand with a damp cloth.

Window frames and glazing compound: Old wood window frames with deteriorated glazing compound will lose that compound under any significant pressure. Once the glazing is gone, water enters the wood behind the glass and causes rapid deterioration. Clean windows by hand with a soft-bristle brush and a diluted cleaning solution.

Decorative trim and millwork: Intricate millwork with narrow grooves and thin sections can be damaged by direct pressure washing. Use a wide-angle tip held at distance, or switch to a garden hose with a spray nozzle on these areas.

Caulked joints: Existing caulk that is still adhered and not yet ready for removal can be dislodged by direct pressure at close range. After washing, inspect all joints and re-caulk any that were loosened before painting.

For these sensitive areas, cleaning with a TSP substitute solution works effectively. Mix one quarter cup of TSP substitute per gallon of water for general cleaning. Apply with a soft-bristle brush, scrub gently, and rinse with a garden hose.

How to Handle Mold and Mildew Found During Power Washing

Mildew on exterior siding requires chemical treatment, not just mechanical removal. Pressure water alone dislodges the visible surface growth but leaves spores embedded in the surface that will regenerate within weeks under the right moisture conditions.

The most reliable treatment approach uses a Jomax-based solution. Mix 80 percent water, 15 percent household bleach, and 5 percent Jomax concentrate. One gallon of Jomax concentrate makes 20 gallons of finished solution. Jomax contains ChlorRelease technology, a bleach activator, plus surfactants that help the solution penetrate into porous surfaces where spores hide. Mix the solution in the correct order: water first, then bleach, then Jomax. Adding the components out of sequence reduces effectiveness.

Apply the solution to mildewed areas and allow it to dwell for 5 to 10 minutes. For light growth on smooth surfaces, five minutes is sufficient. For heavy mildew on rough-textured stucco or wood, extend to 10 minutes and use a stiff-bristle brush with light mechanical scrubbing. The solution remains effective for about three hours after mixing, so do not prepare it in large batches that will sit unused.

Rinse the treated areas thoroughly before the solution dries. Dried bleach residue left on the surface can affect paint adhesion and cause discoloration.

For particularly heavy infestations on areas that get limited sunlight, a second application of the Jomax solution applied after rinsing the first pass provides significantly better results. After the second rinse, allow the surface to dry fully before proceeding.

An alternative for states where TSP is restricted is to use a TSP substitute mixed with bleach at one quarter cup substitute plus one quart bleach per gallon of water. This formulation targets mildew effectively without the phosphate content that makes standard TSP illegal in some jurisdictions.

After treating mildew, inspect the cleaned surface for any soft or spongy wood beneath. Active mildew penetrates wood grain and weakens the fibers over time. Soft spots indicate wood that should be replaced or stabilized with a penetrating hardener before painting, not simply primed over.