Koi Pond – Rendering the pond

Rendering the pond

The next step after you’re done building up your walls, the layout of pipings and angling and smoothing out the corners, is to render the surface of the walls, base, etc.

  • Mix three parts sand to one part cement for the render coat.
  • Apply first coat directly onto the block work. Complete the surface, then rough it up with a trowel to prepare the surface so that the second coat will adhere to the first.

The second coat is made up to the same mix as well as a waterproofing agent and Fibromix. Follow the instructions as per required by the manufacturer. Fibromix consists of glass fibres (so be careful when using this). It should be broken up and added to the mix. The Fibromix serves to give stronger surface, and minimises surface cracking.

  • Apply this second coating mixture on top of the first. Allow to dry.
  • Smooth the surface with a blow lamp with a yellow flame (or a cigarette lighter) by burning any fibres that stick out, then rub the surface smooth. You must ensure that you have a smooth finish.
  • Use a pond sealer after to finish the surface. Follow the pond sealer manufacturer’s instructions carefully.
  • Example of a pond sealer is the G4 pond sealer. It is a moisture-cured polyurethane which forms a non-porous seal on concrete rendered ponds.
  • Method of application
  • Apply the first coat of G4 with a brush, and allow the layer to dry to a finger-tacky point or dry enough to walk upon carefully. This would be approximately 45-60 minutes.
  • Do the same with the second and third coating. Do not allow more thatn 4 hours between coats. G4 comes in Standard (brown translucent) and Mid Green – Black colour. So it you decide to use colour, apply 2 coats of standard and 2 coats of colour.
  • Allow to cure for 72 hours. After that, wash down the pond with plenty of water, drained, and then be fill.


  • G4 Standard – 3 coats = 550g per sqm for the three coats
  • G4 Coloured – 2 coats of standard = 410g per sqm for 2 coats; and 2 coats of colour = 400g per sqm for 2 coats