Acid washing (also called acid etching) is an effective way to deep-clean concrete surfaces, remove efflorescence, mineral stains, and old sealers, and prepare concrete for resealing or coating. Done correctly with the right safety precautions, it's a job many NZ homeowners can tackle themselves — but it does involve hazardous chemicals, so preparation and protective gear are non-negotiable.
What Is Acid Washing and When Do You Need It?
Acid washing uses a diluted acid solution — most commonly hydrochloric acid (also called muriatic acid) — to react with and dissolve mineral deposits, efflorescence (the white powdery residue caused by salts migrating to the surface), rust stains, and concrete laitance. It's commonly used on:
- Driveways and paths before resealing
- Pool surrounds and patios
- Garage floors before applying epoxy coatings
- Any concrete surface with stubborn mineral or calcium build-up
If your concrete just has general grime or mould, a pressure wash with a suitable cleaner may be all you need. Acid washing is best reserved for mineral-based staining or surface preparation.
Safety First — This Is a Hazardous Chemical
Hydrochloric acid is corrosive and produces fumes that can damage your lungs and eyes. Before you start, read the product safety data sheet and follow WorkSafe NZ guidelines for working with hazardous substances. You will need:
- Chemical-resistant gloves (nitrile or neoprene)
- Safety goggles or a face shield (not just glasses)
- Acid-resistant boots or overshoes
- Old clothing that covers your arms and legs
- A P2 respirator rated for acid fumes — a dust mask is not sufficient
- Plenty of clean water nearby for rinsing in case of skin contact
Always work outdoors or in a very well-ventilated area. Never mix acid with bleach or other chemicals.
What You'll Need
- Hydrochloric acid (available at Bunnings or Mitre 10 — look for pool acid or concrete cleaner products)
- Large plastic watering can or plastic pump sprayer (never metal)
- Stiff-bristled brush (plastic or acid-resistant)
- Garden hose with good water pressure
- Baking soda or garden lime (to neutralise the acid)
- Plastic sheeting to protect nearby plants and surfaces
Step-by-Step: How to Acid-Wash Concrete
1. Prepare the Area
Remove all furniture, vehicles, and items from the area. Cover nearby garden beds, plants, and any metal fixtures with plastic sheeting — acid runoff will damage them. Wet down surrounding garden soil thoroughly before you start; this helps dilute any accidental splashes.
2. Pre-Wet the Concrete
Thoroughly wet the entire concrete surface with clean water before applying acid. This is important — applying acid to dry concrete causes it to absorb too quickly and can result in uneven etching or damage.
3. Mix the Acid Solution
Always add acid to water — never water to acid. A typical dilution ratio for general cleaning is 1 part hydrochloric acid to 10 parts water. For heavy efflorescence or etching before coating, you may use up to 1:5, but start weaker and increase only if needed. Mix in a plastic bucket or watering can.
| Purpose | Dilution Ratio (Acid:Water) |
|---|---|
| Light cleaning / efflorescence | 1:10 |
| Moderate staining | 1:7 |
| Surface etching before coating | 1:5 |
| Heavy build-up (use with caution) | 1:4 |
Always check the product label — some pre-mixed concrete cleaners available at Bunnings or Mitre 10 come ready to use.
4. Apply the Acid Solution
Apply the diluted acid evenly across the damp concrete using your plastic watering can or sprayer. Work in manageable sections of around 3–4 square metres at a time. You'll see fizzing and bubbling — this is the acid reacting with the concrete minerals and is completely normal.
5. Scrub the Surface
Using your stiff plastic brush, scrub the surface while the acid is still active (usually within 5–10 minutes). Don't let the acid dry on the surface.
6. Neutralise the Acid
This step is critical. Mix baking soda or garden lime with water (roughly a cup per 10 litres) and apply it over the treated area. You'll see more fizzing as the acid is neutralised. This prevents the acid from continuing to etch the concrete or affecting drainage systems.
7. Rinse Thoroughly
Flush the entire area with plenty of clean water. Rinse at least twice. Be mindful of where the runoff goes — avoid letting it drain into stormwater systems or waterways. Check your local council's guidelines on disposing of chemical waste water.
8. Allow to Dry Completely
Leave the concrete to dry for at least 24 hours before applying any sealer or coating. In NZ's more humid regions (Northland, West Coast), allow longer drying time.
Tips for NZ Conditions
- Avoid acid washing on hot, sunny days — the acid evaporates too quickly and fumes are worse. Early morning on a mild, overcast day is ideal.
- Coastal concrete often has more salt-related efflorescence; you may need a second application.
- If you're preparing concrete for a sealer, check the sealer manufacturer's recommendations — some products like those from Ardex or Sika specify surface preparation requirements.
When to Call a Professional
Consider hiring a qualified concreter or surface preparation specialist if:
- The concrete is heavily damaged, cracked, or structurally compromised
- You're dealing with a large commercial or industrial area
- The staining is from oil, paint, or chemicals that may need specialist treatment
- You're uncomfortable working with hazardous chemicals
- The concrete is part of a pool structure — incorrect acid use can damage pool finishes and affect water chemistry
A professional concreter will have the right equipment, experience, and knowledge of NZ Building Code requirements (particularly MBIE Building Performance guidance on surface treatments). You can find qualified local concreters on tradedirectory.co.nz.
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