Pressure washing is the step most homeowners think of first when preparing concrete for resurfacing, and it’s genuinely important, but it’s also the step that’s most commonly done incorrectly when homeowners try to DIY the prep before a contractor arrives. The gap between a domestic pressure wash and a professional surface clean is larger than most people expect, and doing it wrong can actually make the surface worse to coat. This guide covers what professional pressure washing before resurfacing actually involves, and how it differs from what a domestic machine can achieve. For the DIY approach to pre-clean, see our companion guide on pressure cleaning before resurfacing DIY.
Quick Answer (BLUF)
Professional pressure washing before resurfacing uses commercial-grade equipment (3,000-5,000 PSI) with surface cleaner attachments, preceded by a degreasing step, and is followed by adequate drying time. It removes loose material, contaminants, and surface contamination that would prevent overlay adhesion. A domestic pressure washer at 1,200-1,500 PSI with a lance alone cannot achieve the same result, it moves dirt around more than it removes it.
Why the Pre-Wash Matters So Much
Concrete surfaces accumulate contamination that isn’t visible to the naked eye: atmospheric oils, diesel dust from vehicle traffic, algae and moss spores, chemical residues, and old sealer. All of these reduce adhesion. When an overlay bonds to contamination rather than to the concrete itself, the overlay eventually separates at the contamination layer, which can look like the overlay failed when really the prep failed.
On the Central Coast, driveways also accumulate:
- Coastal salt deposits: Crystallised salt in the concrete’s pores needs to be flushed out before overlaying. Sealing salt into the surface causes osmotic pressure that lifts coatings.
- Organic growth (algae, lichen, moss): Extremely common on shaded driveways in the humid coastal climate, particularly in Kincumber and Erina where trees shade driveways. Algae roots penetrate into the concrete surface and must be killed and removed, not just visually cleared.
- Rubber tyre deposits: Bitumen and rubber residue from repeated parking in the same spots, particularly significant in commercial and rental properties in Gosford.
The Professional Pre-Wash Process
Step 1: Pre-Treat with Degreaser
Before any water goes on the surface, contamination type is assessed and appropriate chemical pre-treatment is applied:
- Oil and grease contamination: A concrete degreaser (alkaline surfactant) is applied, worked in with a stiff brush, and allowed to dwell for 5-15 minutes. This emulsifies the oil, allowing it to be flushed away.
- Algae and organic growth: A diluted sodium hypochlorite or quaternary ammonium biocide solution is applied, dwell for 10-20 minutes to kill the organism, then pressure washed off. Dead algae washes off cleanly; live algae can smear and leave residue.
- Efflorescence: Diluted hydrochloric acid (1:10) applied to affected areas to dissolve the calcium carbonate deposits.
Why this step is skipped by DIYers: Chemical pre-treatment takes time, requires appropriate products, and involves chemicals that require careful handling. Skipping it and going straight to pressure washing disperses contamination without removing it.
Step 2: Professional-Grade Pressure Washing
Commercial pressure washers used by professional concrete contractors operate at 3,000-5,000 PSI, 2-4× the output of a typical domestic unit. More importantly, the flow rate (litres per minute) is significantly higher, meaning more water volume to flush contamination away rather than just pressurising it.
The surface cleaner attachment is critical for flat surfaces. A surface cleaner is a circular housing with two or three rotating nozzles inside, it delivers consistent, overlap-free cleaning pressure across the surface without the streaking pattern that a lance creates. Professional surface cleaners are 45-60 cm wide, dramatically faster than a lance on large flat areas.
Step 3: Sealer and Paint Removal (Where Required)
If existing sealer is flaking or poorly adhered, a pressure wash alone won’t remove it adequately, it needs to be ground off mechanically. However, water-based acrylic sealers in reasonable condition can sometimes be removed (or at least stripped back to a thin, stable layer) with hot water pressure washing at appropriate pressure.
Old sealers that are stable and well-adhered can sometimes be overlaid directly, provided the overlay system is compatible. A professional contractor will assess this during the site visit.
Step 4: Drying Time
After pressure washing, the slab must dry completely before any overlay, coating, or primer is applied. Professional contractors allow:
- 24 hours minimum for warm, sunny, good-airflow conditions
- 48-72 hours in winter or shaded positions
- Moisture test before applying epoxy or any coating with low moisture tolerance
Rushing the drying step is particularly risky on the Central Coast where coastal humidity keeps slab moisture elevated for longer than it would in drier inland conditions.
What a DIY Pressure Wash Achieves (and Doesn’t)
A domestic pressure washer (1,200-1,800 PSI with a lance) will:
- Remove loose surface dirt and dust
- Dislodge some loose flaking material
- Clear visible organic growth (but not kill or remove the roots)
It will not:
- Remove embedded oil contamination (without prior degreasing)
- Achieve consistent cleaning pressure without streaking
- Match the flushing power of 4,000 PSI + high flow rates
A DIY pre-wash is better than nothing, it reduces the contamination load a contractor has to deal with. But it should be understood as a preliminary clean, not a substitute for professional prep.
FAQs
Should I pressure wash my driveway the day before the contractor arrives?
If you have access to a pressure washer and can do a basic clean, it helps. But tell the contractor so they know the slab has been recently wetted and can allow adequate dry time before applying any coating.
The contractor says pressure washing is included in the job, what exactly are they doing?
Ask specifically: what PSI machine are they using, do they use a surface cleaner attachment, do they pre-treat for oil/algae, and how long are they allowing for drying before the overlay goes down? This tells you whether you’re getting a professional prep or a quick rinse.
Is hot water pressure washing better than cold?
Yes for oil contamination, hot water dramatically increases the effectiveness of degreasing, as heat emulsifies oils more effectively than cold water. Professional hot water units are more expensive and less common, but significantly better for heavily contaminated commercial and workshop floors.
Can pressure washing damage my concrete?
Domestic pressure washers rarely damage sound concrete. Professional units at 4,000+ PSI can damage old, weak, or spalling concrete surfaces, which is actually useful diagnostic information. If the pressure wash reveals delaminating surface concrete, that material needs to be removed anyway before any overlay is applied.