Concrete is a surface with a paste layer on top, and that layer is what looks clean when it is intact and looks permanently dirty once it has been blasted away. Getting the nozzle, the chemistry and the sequence right matters more than pressure.
Pressure, chemistry and sequence
General concrete cleaning is pressure washing with the right nozzle and a surface cleaner rather than a wand, plus a degreaser for oil and a mild acid for efflorescence. Too much pressure etches the surface and strips the top paste, which makes it hold dirt faster.
Prices are Freshway catalogue prices as at 16 September 2026 and are subject to change. Confirm the current price, and whether GST applies, before quoting.
Match the method to the soiling
| Soiling | Method | Note |
|---|---|---|
| General dust and grime | Pressure wash, 25° nozzle, with a surface cleaner on slab areas | Water alone is usually enough |
| Oil and grease | Alkaline degreaser with dwell time, then pressure wash | Work from the outside of the stain inwards |
| Efflorescence (white salt bloom) | Mild acid wash, then rinse thoroughly | It will return if moisture is still migrating |
| Moss, algae and black spot | Pressure wash, then a residual treatment | Cleaning alone does not stop regrowth |
| Paint and coatings | Chemical stripper or mechanical removal | Not a pressure washing job |
The nozzle rule that prevents most damage
Pressure is a function of nozzle angle and distance, not just the machine rating. A zero-degree jet held close to concrete will cut a groove in seconds; the same machine with a 25° nozzle and a surface cleaner cleans a driveway evenly. For commercial work the surface cleaner is the tool that makes the job both faster and safer for the slab.
Two practical limits: keep the wand moving, and never hold the jet on a joint, a drain edge or a repair patch, where the pressure finds the weakest point first.
Equipment we stock for the job
| Item | Use | Price |
|---|---|---|
| Karcher HD 5/12 C Plus | Commercial cold water machine, 2538 psi class | A$1,649.00 |
| DEWALT 457 mm surface cleaner | Even coverage on slabs and driveways | A$295.00 |
| Turbo nozzle, 4500 psi | Heavy soiling on sound concrete | A$145.90 |
| Heavy Duty Degreaser 395 | Oil and grease pre-treatment | A$60.00 |
Why does white vinegar seem to work?
Vinegar is a weak acid, and acid does dissolve mineral deposits, which is why it lifts efflorescence slightly and does nothing to oil, moss or general grime. A purpose-made acid wash is stronger, cheaper per square metre and predictable, and it can be neutralised and rinsed properly so it does not attack the concrete itself.
Making concrete look new again
Cleaning reaches a limit on concrete that has stained through: the honest options are a surface preparation (grinding or shot blasting) followed by a sealer, or decorative resurfacing. Where the concrete is sound but the colour is inconsistent, sealing after cleaning is what holds the improvement and makes the next clean easier.
Wash water matters too. Pressure washing a car park sends oil, detergent and sediment somewhere, and the NSW Environment Protection Authority covers chemical and wash water controls. On commercial sites, capturing and dealing with the run-off is part of the job, not an afterthought.
Sealing and protecting after the clean
Concrete that has just been cleaned is concrete at its most absorbent, which makes it the right moment to seal. A sealer slows the penetration of oil, water and salt, and on a driveway or workshop slab it turns the next spill into a wipe rather than a treatment. The surface has to be dry first; applying sealer to damp concrete traps moisture and shows as a milky patch.
Where a slab is also a work surface, check the slip requirement before sealing: some film-forming sealers reduce the wet slip resistance, which matters on a ramp, a loading dock or any area that gets hosed. A penetrating sealer is the safer choice where foot traffic is heavy.
Questions we get asked most
- What’s the best thing to clean concrete with?
- Water and pressure for general soiling, an alkaline degreaser for oil, and a mild acid for efflorescence. The equipment matters as much as the chemistry: a surface cleaner gives even results where a wand leaves stripes.
- How to make concrete look new again?
- Clean it properly first, then seal it. Where the surface is stained through or worn, cleaning reaches a limit and the options are surface preparation and sealing, or resurfacing.
- Does white vinegar clean concrete?
- It softens light mineral bloom because it is acidic. It does not remove oil, moss or general grime, and on large areas it is slower and more expensive than a formulated acid wash.
- What not to clean concrete with?
- Do not use undiluted strong acid, do not leave acid to dry on the surface, and do not use a zero-degree jet at close range. And do not let the run-off go to stormwater: it is a trade waste problem.
- Can I pressure wash a new concrete slab?
- Wait until it is fully cured — generally 28 days — and then use a lower pressure with a surface cleaner. New concrete has a softer paste layer and marks easily.
References
- NSW Environment Protection Authority — chemicals and wash water.
- SafeWork NSW — machinery and equipment.
Concrete that needs more than a wash?
Tell us the area, the soiling and whether the slab is sealed, and we will spec the machine, the nozzle and the chemistry for it. Browse pressure washers or ask our team.