Guide · DY Exterior Cleaning Ltd
How to clean render and K-Rend properly (without ruining it)
Clean K-Rend and modern thin-coat render using soft-wash methods only, apply a sodium hypochlorite biocide at low pressure, let it dwell 20,40 minutes, then rinse with pure water at low pressure. Pressure washing K-Rend strips the silicone finish and opens the surface to water damage.
Why render goes green
Modern thin-coat render is fertile ground for algae, moss and red lichen, especially north-facing walls, shaded corners, and walls below leaky gutters. The growth doesn't damage the render directly, but it looks awful and the spores spread fast once established.
The correct method: soft wash
Apply a sodium hypochlorite based biocide at very low pressure, usually a 12V pump or gravity sprayer. Let it dwell 20,40 minutes. Rinse off at low pressure with pure (deionised) water. The biocide kills algae at root; over the following weeks the dead growth washes away and the original render colour returns.
Why pressure washing K-Rend ruins it
K-Rend is a silicone-modified through-coloured render. Pressure water scours the silicone surface, removes the integral water-repellent layer, and opens the substrate to long-term water ingress. Once damaged it cannot be repaired without re-rendering.
Monocouche and silicone render
Monocouche (Parex, Weber) and acrylic silicone systems behave similarly, soft wash only. Pebble dash and traditional sand-and-cement render are more robust and can take controlled pressure cleaning, but soft wash is still safer.