Acid Washing

Removing Lime-scale with Acid

Acid-washed Steps

Acid-washed Steps - Before and After

If you have allowed the pH of the pool water to get too high, have left your pool to its’ own devices over winter (a very bad idea!) or have inherited a problem pool it is probable that you will have a build-up of lime-scale on the sides and bottom of the pool

Lime-scale is formed from minerals that are dissolved in the pool water and it unfortunately provides the perfect toe-hold for algae to grow and proliferate

Lime-scale feels rough to the touch because it is composed of millions of microscopic crystals with ‘pits’ (holes) on the surface.  These pits are circulation dead-spots and do not receive fresh supplies of chlorinated water

Algae spores that takes up residence in one of these pits are unaffected by normal levels of chlorine and can therefore proliferate, turning the lime-scale green

Superchlorination will probably, but not definitely, kill the algae and turn it brown.  Now you have unsightly, brown lime-scale on the pool surfaces

Scrubbing with a stiff brush does little to remove lime-scale once it has developed past a certain point and the only choice left is to drain and acid-wash the pool

This is a very unpleasant process and should not be undertaken lightly.  Our sincere advice is to get a professional in to do the job for you but if you really want to have a go yourself - this is what you do: -

Instructions for Acid Washing

  • Wear rubber boots and gloves, eye protection, long trousers and a shirt with sleeves (wear old clothes and footwear
  • Wear a respirator with the correct acid-fume-absorbing filters. THIS IS IMPORTANT to prevent damaging your lungs
  • Drain the pool to ‘Waste’, but leave about 200mm (8”) of water at the deep end (to throw yourself into if you splash yourself with acid)
  • Do not use the pool water to backwash a sand-filter whilst draining the pool, as there is a strong possibility that the filter will calcify as the water passes through it. Then you might need to replace the filter!
  • Have a hosepipe ready to rinse off each section of the pool as it is cleaned
  • Pour 5 litres of water in a rubber bucket and slowly add 5 litres of Hydrochloric (Muriatic) Acid, use all of the mixture before mixing more
  • Add the acid to the water; NEVER add the water to the acid
  • Using a long-handled soft broom, apply the mix to the top of the pool wall in a sweeping motion
  • Do not allow the acid to contact any stainless-steel fittings such as steps or handrails as they will become stained
  • Apply the acid with a soft broom at arms lengthApplying acid with a broom
  • Cover about 1 metre width of wall at a time from the top and allow the acid to drip down to the bottom
  • Sweep the acid across the tiles with the broom - it's the acid and not scrubbing with the broom that removes the lime-scale - and keep sweeping it upwards from where it has dripped to minimise the amount of acid required
  • The acid will hiss and froth as it dissolves the lime-scale (and the top surface of the grout/plaster) and it will be obvious where lime-scale is still present
  • Work quickly to minimise grout or plaster damage
  • As the acid removes the lime-scale from each section, rinse that section thoroughly
  • Pay particular attention to washing away the ‘run-off’ on a regular basis because it is very acidic and will dissolve the grout/plaster as it drains into the bottom of the pool
  • When all the walls of the pool have been washed and thoroughly rinsed, start on the steps and the bottom surface of the pool at the shallow end. Take extra care when entering or leaving the pool via the steps; once they have been acid-washed they may be very slippery
  • Continue cleaning until all visible lime-scale has been removed
  • Drain the remaining water to ‘Waste’ and remove any lime-scale that still remains
  • Rinse the entire pool surface thoroughly and check for remaining patches of lime-scale
  • Any patches that you missed will become more obvious as the surface dries
  • Check for any areas of eroded or missing grout or damaged plaster and repair as required


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Ken Walker - MyPoolGuru©