Introduction

If your wash prep is sloppy, your paint job will tell on you in a few months. Peeling over chalked siding, fuzzy wood grain from too much pressure, streaks under trim—most of it starts with the wash. Here’s the why and how: you’re removing contaminants, opening the surface, and setting the pH so primer actually bonds. Below are practical, job-proven best practices that keep you fast and safe on vinyl, wood, fiber cement, stucco, and brick—without buying a warehouse of gear.

Table of Contents

• Key Takeaways
• Diagnose Before You Spray
• Equipment and Settings That Protect Surfaces
• Control Chemistry: Detergents, Bleach, and pH
• Manage Water, Runoff, and Neighbor Impact
• Drying, Inspection, and Pre-Paint Repairs
• Execution Playbook: A Fast, Repeatable Flow
• Frequently Asked Questions
• Conclusion

Key Takeaways

• Use flow (GPM) and nozzle angle to do the work—reserve high PSI only for durable substrates.
• Match chemistry to the problem: surfactant for dirt, sodium hypochlorite (SH) for organics, oxalic/citric for rust/tannin.
• Control runoff and overspray up front; pre-wet plants and protect fixtures to avoid damage claims.
• Don’t paint over wet or high-pH surfaces—target less than 15% wood moisture and neutral pH before priming.
• A simple, repeatable wash sequence reduces callbacks and keeps crews consistent.

Diagnose Before You Spray

The Problem

Many contractors jump straight to blasting, then chase streaks, fuzzed wood, and missed algae. Skipping diagnosis wastes time and can damage surfaces. It’s common for crews to miss chalking, oxidized siding, and hairline cracks that will telegraph through paint.

The Solution

• Walk the perimeter and call out substrate, condition, and risks.
• Do quick tests:
  • Tape pull for chalking on painted siding.
  • Moisture meter baseline on suspect wood—note anything already above 18%.
  • Probe caulk joints and trim end-grain for rot.
• Decide “pressure wash” versus “soft wash.” In general, delicate or aged coatings prefer soft wash (low pressure plus chemistry).

Real-World Example

1960s painted cedar with heavy chalk: Tape test leaves a white print. Plan a detergent pre-wash, gentle rinse with 25–40° tip at roughly 600–800 PSI at the surface, then a post-wash check. No aggressive fan close to the grain.

Equipment and Settings That Protect Surfaces

The Problem

Too much pressure etches concrete, digs into wood, and forces water behind lap siding. Not enough flow leaves you chasing dirt lines. Contractors often over-rely on PSI and underuse GPM and tip size.

The Solution

• Prioritize GPM: A 2.5–3.0 GPM unit commonly cleans faster and gentler than a higher-PSI, low-flow unit.
• Set PSI by substrate:
  • Vinyl/fiber cement: typically 1,300–1,600 PSI at the surface.
  • Painted wood: commonly 500–1,200 PSI; keep 6–12 inches off the surface.
  • Brick/masonry: 1,500–2,500 PSI, test a small area first.
• Use proper nozzles and distance; let the fan angle and flow do the work.
• Avoid turbo nozzles on coated wood or old paint—save them for durable masonry when needed.

Nozzle and Surface Pairings

Real-World Example

Fiber cement with pollen and mildew: 40° fan, roughly 1,400 PSI at 8–10 inches, paired with a 2.8–3.0 GPM machine. The wider fan reduces lap intrusion, the flow lifts debris. Result: clean substrate without forcing water behind boards.

Control Chemistry: Detergents, Bleach, and pH

The Problem

Pressure alone won’t kill mildew roots or break down chalk efficiently. Overusing bleach can burn plants and leave high pH that hurts adhesion. Underusing it leaves organics alive under paint.

The Solution

• Match chemistry to contaminants:
  • General dirt/pollen: Mild detergent/surfactant.
  • Mildew/algae: Sodium hypochlorite (SH) at about 0.5–1.0% on siding; up to 2–3% on stubborn growth. Always pre-wet and protect vegetation.
  • Rust/tannins/iron bleed: Oxalic or citric acid cleaners to brighten and neutralize.
• Dwell, then rinse: Let solutions sit 5–10 minutes out of direct sun; don’t let them dry. Reapply rather than cranking up PSI.
• Neutralize when needed: After strong SH or alkaline cleaners, a light acidic rinse (oxalic/citric per label) helps bring pH closer to neutral before primer. Many paint manufacturers expect a near-neutral substrate for best adhesion.

Real-World Example

North-facing vinyl with green growth: Apply 1% SH with surfactant, 7-minute dwell in shade, gentle agitation with a soft brush, then rinse with a 40° tip. Organics release without gouging the surface.

Manage Water, Runoff, and Neighbor Impact

The Problem

Overspray fogs windows, stains fixtures, and unsettles neighbors. Runoff can carry soap, solids, and diluted SH into storm drains. It’s common for callbacks to be about landscaping or mess, not the paint.

The Solution

• Protect and pre-wet:
  • Soak plants with clean water before and after any SH use; water dilutes uptake.
  • Cover exterior outlets, door thresholds, and light fixtures.
  • Shut windows; ask clients to keep them closed until dry.
• Control runoff:
  • Use gutter downspout extensions to steer water to lawn, not hardscape.
  • Block storm inlets where practical; pump or divert to turf where allowed.
  • Local stormwater rules commonly prohibit chlorinated or sediment-laden wash water entering storm drains—check your jurisdiction.
• Work top-down, leeward to windward: Reduce overspray drifting to clean areas or neighboring properties.

Real-World Example

Tight-lot home with a sidewalk drain: Set inlet protection, route downspouts into lawn with extensions, and assign one helper as “plant guard” to mist shrubs during SH dwell. End-of-day, remove inlet protection and bag debris.

Drying, Inspection, and Pre-Paint Repairs

The Problem

Painting over wet, chalky, or high-pH surfaces is a fast track to early failure. Many contractors rush the dry time, especially in cool or humid weather.

The Solution

• Respect drying windows: In general, allow 24–48 hours after washing before primer, longer in shade or high humidity.
• Verify moisture: Many coatings call for wood moisture less than 15% before primer. Use a pin meter on end grain and suspect boards.
• Check pH after aggressive cleaning on masonry: Aim near-neutral; if in doubt, a mild acidic rinse can help.
• Inspect for damage revealed by washing: Soft wood, failed caulk, popped nails. Fix now—don’t trap problems under paint.

Real-World Example

Stucco priming plan: After alkaline cleaner, use a pH test strip; reading remains high. Apply a light oxalic/citric rinse, retest to near-neutral, then schedule primer the next morning once surface is dry.

Execution Playbook: A Fast, Repeatable Flow

The Problem

Inconsistent wash steps lead to missed areas and callbacks. New helpers often over-spray and under-rinse.

The Solution

Use a simple SOP that crews can follow on every exterior paint prep:

1. Walkthrough and callouts: Substrates, risks, plants, neighbors, runoff plan.
2. Protect and stage: Cover fixtures, pre-wet plants, set downspout extensions, block drains if needed.
3. Dry clean first: Broom/brush cobwebs, loose debris—reduces chemistry waste.
4. Apply chemistry by zone: Shade-first; use 0.5–1.0% SH for organics, detergent for dirt; 5–10 minute dwell.
5. Agitate selectively: Soft brush stubborn spots; avoid scuffing coatings.
6. Rinse from top down: 40° tip where possible; keep 6–12 inches off siding; follow laps.
7. Detail rinse: Windows, fixtures, door hardware; final plant rinse.
8. Drying: Leave windows cracked open later? No—keep closed until surfaces dry; schedule 24–48 hours before primer.
9. Verification: Moisture meter checks on wood; pH spot check on masonry if applicable.
10. Punch list and photo record: Document any repairs found—soft boards, caulk failures, hairline cracks.

Real-World Example

Two-person crew on a 1,800 sq ft two-story: Following the SOP, they wash in four elevations, clock a 7–8 minute chemistry dwell, and use a 3.0 GPM washer at moderate PSI with 40° tip. With plant guarding and downspout extensions, they finish wash and site tidy in about half a day, return next morning to prime after moisture checks.

Frequently Asked Questions

How Long Should I Wait To Paint After Washing?

In general, allow 24–48 hours depending on sun, shade, and humidity. Don’t go by feel—check wood with a moisture meter and target less than 15% before priming. Masonry can hold moisture longer; adjust accordingly.

Can I Power Wash Surfaces With Old Lead Paint?

Don’t blast lead-painted surfaces. Use lead-safe practices (containment, minimal disturbance, HEPA cleanup) and follow applicable regulations in your area. Soft cleaning and hand methods with proper containment are safer. If in doubt, test before you plan the wash.

Soft Wash Or Pressure Wash—Which Is Better?

Use chemistry to do the work whenever possible. Soft wash (low pressure plus the right cleaner) is safer on aged coatings and wood. Reserve higher pressure for durable masonry and heavy buildup after testing. Many contractors find a 40° tip and moderate PSI handle most siding when paired with proper chemistry.

What SH Mix Is Safe Around Landscaping?

Keep siding mixes around 0.5–1.0% SH for general organics, up to 2–3% for stubborn spots. Always pre-wet plants, avoid pooling, and rinse thoroughly after. Work in shade to reduce flash-dry and plant stress.

Do I Need To Neutralize After Using Bleach?

If you used higher-alkaline cleaners or strong SH, a light oxalic/citric rinse helps bring pH near neutral—especially on masonry—before primer. Follow product labels and your paint manufacturer’s guidance.

Conclusion

Prep is where exterior paint jobs are won or lost. Diagnose first, let flow and chemistry do the work, control runoff, and don’t rush drying or pH. Document what you’ll wash, what chemicals you’ll use, and how you’ll protect the site—clients notice that clarity. If you want to capture site notes fast while you’re still curbside, talk, snap, and send a clear scope with Donizo. Voice to Proposal lets you record conditions and photos and generate a professional PDF; clients can e-sign, and you can convert accepted proposals to invoices in one click. Less admin, more paint on walls.