I have been asked to clean the render and concrete window sills on a house, we have cleaned several exteriors before, but all have been painted so fairly easy to clean. This job isn't painted render, i cant tell you exactly what it is but it seems be kind of like rough sandstone, its quite porous so cleaning with our pole system is pointless (apart from ruining the brushes!). Is there any chemicals people can recommend or would pressure washing be an option. TIA.