Pressure Washing Prep (PSI and Tip Selection by Substrate)
Why this matters
The wrong pressure tip on the wrong substrate ruins the substrate, the paint job, and the customer's house. A 0 degree red tip held three inches from cedar siding will route a furrow through the wood that no caulk or primer can hide. A 40 degree white tip held two feet from heavy chalking stucco will not move the chalk and the paint will fail in eighteen months. PSI selection and tip selection are the two knobs a painter has to balance: enough energy to remove the contamination, not so much that the substrate is damaged. This is the field card.
What pressure washing is doing
The job of pre-paint pressure washing is removing four things: dirt, loose paint, chalk, and biological growth (mold, mildew, algae). It is not stripping paint to bare wood; that is what chemical stripping or abrasive blasting is for. It is preparing a clean, sound, paint-ready surface.
If the washer is removing the substrate (raising wood grain, eroding stucco aggregate, blasting mortar from masonry joints), the pressure is too high or the tip is too narrow.
PSI and tip basics
A pressure washer has two outputs: pressure (PSI) and flow (GPM). Pressure does the cleaning energy; flow rinses the loosened material away. Most residential gas-powered units run 2,800 to 4,200 PSI at 2.5 to 4.5 GPM. Industrial units run higher PSI but the substrate-damage risk scales with it.
The tip on the wand controls fan angle and converts pressure into a cleaning pattern:
| Tip color | Fan angle | Use |
|---|---|---|
| Red (0 degree) | 0 degree pinpoint | Never use on a paint-prep job. Damages every substrate. |
| Yellow (15 degree) | 15 degree narrow fan | Concrete, metal, hard surfaces only. Too aggressive for siding. |
| Green (25 degree) | 25 degree medium fan | Heavy soil on durable substrates: concrete, brick, cement board, aluminum siding |
| White (40 degree) | 40 degree wide fan | Wood siding, vinyl siding, stucco, painted surfaces with sound paint |
| Black (65 degree) | 65 degree wide fan, low pressure | Soap application, rinse, delicate substrates |
Distance from the substrate is the other variable. Doubling the distance halves the effective impact pressure. Working too close compensates for using too wide a tip but introduces gouging and overspray patterns.
Substrate-by-substrate procedure
Wood siding (cedar, redwood, pine)
- Tip: white 40 degree
- PSI at the substrate: 1,200 to 1,500 effective. Pull the wand back to 18 to 24 inches if the rated PSI is higher.
- Direction: spray with the grain, not against. Spraying across grain at high pressure raises grain and shreds soft woods.
- Watch for: mill glaze on new cedar (does not wash off; requires sanding), grain raise (back off pressure), water entry behind siding (work down, not up).
Allow 48 to 72 hours drying before primer. Wood holds water; checking moisture content with a pin meter and not painting until below 15 percent is the protocol that survives a year-end callback.
Vinyl siding
- Tip: white 40 degree, or black 65 degree with soap
- PSI at the substrate: 1,000 to 1,500
- Direction: downward angle; do not spray upward under laps (water gets behind the siding and sits)
- Watch for: chalking. Aged vinyl has a chalk layer that has to come off before paint adheres. Use a vinyl wash detergent applied via a soap nozzle and dwell 5 to 10 minutes before rinsing.
Stucco
- Tip: white 40 degree
- PSI at the substrate: 1,000 to 1,800. Lower on textured stucco; higher on smooth.
- Distance: 12 to 18 inches
- Watch for: dislodging hairline cracks (visible after washing because they fill with water). These need to be addressed before paint, with elastomeric patch or with a crack-bridging primer.
Stucco that is heavily chalking needs a soft-wash detergent step. A pure pressure rinse moves chalk around the wall but does not remove it. Use a 1 percent trisodium phosphate (TSP) solution or a commercial chalk-removal detergent before the rinse pass.
Brick (painted or unpainted)
- Tip: green 25 degree on durable, sound brick; white 40 degree on older soft brick
- PSI at the substrate: 1,500 to 2,500 on durable; 1,000 to 1,500 on soft
- Watch for: mortar joint erosion (back off immediately if mortar is blasting out), efflorescence (white salt deposits; needs separate acid wash treatment), spalled brick faces (do not wash spalled areas at full pressure).
Older brick (pre-1930 soft fired) is vulnerable to pressure damage. Test in an inconspicuous area before committing to PSI.
Concrete and CMU block
- Tip: yellow 15 degree on heavy soil; green 25 degree for general cleaning
- PSI at the substrate: 2,500 to 3,500
- Watch for: surface aggregate erosion (back off if losing aggregate), efflorescence
Concrete is the substrate that tolerates the highest PSI. Useful for removing old failed paint, but the painter still needs to verify the substrate is sound after the wash.
Metal and composite siding
Metal (steel, aluminum, painted): yellow 15 degree on durable; white 40 degree on aluminum or thin-gauge; 1,500 to 2,500 PSI. Watch for rust dislodgement (good), aluminum denting (back off), water into seams.
Composite (HardiePlank, LP SmartSide): white 40 degree; 1,000 to 1,500 PSI. Watch for water entry at horizontal laps (spray downward only) and substrate saturation. HardiePlank tolerates pressure better than LP SmartSide.
When to use soap
A pure water pressure wash moves dirt, but it does not kill mold. For any substrate with visible mildew (gray-black biological staining on the north or shaded side of a building, or under tree drip lines), a sodium hypochlorite (bleach) wash is required.
Mix at 1 to 4 ratio of household bleach (5.25 percent sodium hypochlorite) to water for a moderate biological load; 1 to 2 for heavy. Apply through a soap nozzle or via a separate pump sprayer at low pressure, dwell 10 to 15 minutes (do not let dry; re-wet if drying), then rinse with the white 40 degree tip at moderate pressure.
Sodium hypochlorite damages plants and landscaping. Pre-wet all plants below and around the wash area, cover sensitive plantings with tarps, and rinse with clean water immediately after the wash. A customer's mature ornamental bed killed by bleach overspray turns a profitable job into a loss. Plan plant protection before the soap goes in the tank.
Dry time before paint
Pressure washing is not a 30-minute step. The substrate has to be dry before primer or topcoat. Field rules:
- Vinyl siding: 24 hours
- Stucco, brick, concrete: 48 hours minimum, longer in cool or humid weather
- Wood: 48 to 72 hours, verified with moisture meter at 15 percent or below
- Metal: 4 to 8 hours; flash rust the next day means waiting was wasted
Painting over a wet substrate is the most common cause of exterior coating failure within the first 12 months. The job timeline has to allow the dry days; do not let a customer's schedule force a same-day paint after a wash.
References
- ASTM D4259 - Standard Practice for Abrading Concrete
- ASTM D4258 - Standard Practice for Surface Cleaning Concrete for Coating
- Sherwin-Williams Painting Specification Manual (exterior surface preparation chapter)
- Cleaner Times trade publication, pressure washing safety and substrate guides
- Hardie James Hardie installation and finishing best practices, surface preparation