Power washing is great for cleaning many surfaces however it is only one step in the cleaning process. Often you will find that when an area has been power washed the spores from fungi, lichens and so on (biofilms) are displaced which then land on the clean surface and start to recolonise and before you know it the area is looking as dirty as ever. Also be wary of what you power wash. Renders and tar can easily become damaged by power washing at high pressure so care must be taken.