In many occasions we have been asked by clients to render over existing render, one of the obvious benefits of doing this would be that the cost of removing the existing render is eliminated. However there are a few things to consider before you choose to follow through with this option as making the wrong decision can be very costly and stressful.
The first thing to look at is the render and if it is painted? The reasons this is a problem is that if the existing render is painted then it will not be possible for the new render to stick onto the existing render, this is because the paint layers will not allow the new render to bond with the existing render. If you do decide to proceed without removing the paint from the render you will probably notice cracks in the new render within 6 months of having completed the project.
Secondly you need to check if the existing render is cracked or is coming loose, if that is the case then all loose render would need to be removed from the wall before you can proceed with applying the new render. It would be an idea to check if the render is hollow. All hollow areas would need to be removed before you proceed with re-rendering the property.
Finally if you do decide to render over existing render please ensure that you choose to re-render with a lightweight render system rather than traditional sand and cement system. The lightweight render system like a thin coat application from K Rend or SPS Envirowall will enable you to achieve such a finish.