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Draining & Acid Washing

Draining & Acid Washing the surface is done to either remove stains caused by a pool that has been dormant for over a year or to remove significant calcification that has formed on the surface. This can only be done on Concrete & Pebblecrete pools. Do not attempt to do this if you have a Vinyl or Fibreglass surface as serious structural damage can occur (warping/buckling of shell).

1. What you’ll need

Item Why
Submersible pump / hose (rent from equipment hire) Fast drain to sewerage point
PPE: Goggles, respirator, gloves & boots Fumes and splash risk
Hydrochloric Acid, ~20L for an average 8 × 4 m pool Etches stains and scale
Large plastic watering can or acid-safe sprayer Even application
Pool broom or acid-resistant brush Work the solution into plaster
Alkalinity Increaser (baking soda) Neutralise rinse water
Garden hose (high-flow nozzle) or pressure cleaner Fast rinsing
Expansion plugs (typically 2–3 × 40mm) Block returns & main drain


2. Drain the pool (monitor periodically)

  1. Shut off power to equipment (pump, chlorinator, etc).

  2. Drop the submersible pump in (deep-end floor); route hose to a sewerage drain (stormwater drain is illegal).

  3. Stay on-site—shell hydrostatic pressure changes quickly.

  4. Stop when only 2–3cm remains; drag the pump to the shallow end and finish.

3. Prep the surface

  • Rinse walls and floor to knock off loose debris.

  • Block main-drain and return lines (w/ expansion plugs) to keep acid out of plumbing.

4. Mix acid wash (always acid into water)

  • Standard ratio 1 part acid to 4 parts water in the watering can.

  • Make only what you’ll use in ~10 minutes to keep fumes down.

5. Apply & scrub—work in 3m² sections

  1. Starting at the tile line, pour/spray the solution in a consistent snake like pattern.

  2. Let it fizz for 30–60 seconds—no longer; over-etching weakens plaster.

  3. Lightly brush to lift stains and scale; stubborn spots may need a second light pass, never aggressive scraping.

  4. Immediately hose the section down, pushing runoff to the deep end.

6. Neutralise the waste

  • When all walls/floor are done, sprinkle baking soda (≈1 kg per 20L acid used) over the remaining puddle; mix with the broom until fizzing stops.

  • Pump the neutralised slurry to sewer, not garden or storm-water.

7. Rinse twice & unblock lines

  • Hose every surface top-to-bottom.

  • Pull out expansion plugs; hose returns, skimmer and main drain.

8. Refill as soon as possible

  • Start filling immediately to relieve shell stress; aim garden hose at deep end to avoid staining fresh plaster.

  • While filling, check hydrostatic relief plugs—reinstall if removed.

9. Re-balance water

  1. Once the water reaches mid-skimmer, start the pump and circulate 24h.

  2. Adjust alkalinity first, then pH, then calcium; new plaster will naturally raise pH the first week.

  3. Resume normal chlorination after chemistry is stable.

Safety / damage caveats

  • Acid washing removes ~0.25mm of plaster each pass; limit to once every 5–7 years.

  • Never leave an empty pool overnight— it's possible for groundwater to cause the shell of the pool to float.

  • If heavy rain is possible or you see hydrostatic relief valves burping water, stop and consult a professional.

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