Rendering over existing rendered/painted backgrounds

We do not advise rendering over existing renders, site batched renders or other manufactured systems as there are too many unknown anomalies that would prevent an informed judgement. Specifically, these relate to the condition of the existing render in regard to its strength, thickness, quality and existing applied surface treatments to provide recommendation to support the additional extra weight from a new render.

For this reason, we would therefore be unable to predict the lifespan of a further render coat and its potential for cracking or debonding. 

In addition, and more significantly, rendering onto painted surfaces is not in-keeping with good building practice. In this instance, delamination, crazing and cracking are highly likely outcomes in this scenario.

There are three solutions available in this instance, with a further coat of render being the only solution if deemed paramount. However, we will not guarantee a system that has been applied over existing rendered surfaces, other than an identified, virgin, existing Weber render after a detailed inspection has been carried out by a member of our specification team.

Common issues and queries you can face

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Render coats are applied in successive layers

Render coats are applied in successive layers in decreasing thicknesses and strengths. Topcoats therefore may be relatively weak. It is difficult to assess the strength of existing materials and their bond strength to previous layers.

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Stresses from additional materials

Additional materials put increasing stresses on the bond interfaces of existing materials.

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Paint, dirty deposits or organic growth

1. Existing renders are often finished with a thin coating or paint which will form a weak interface which is not suitable for rendering over. 2. Dirty deposits accumulated over a period of time can forma weak intermediate layer that interferes with the development of the bond of newly applied render. 3. Organic growth, even though it may be brushed from the surface, remains in the pores of the substrate and can continue to grow, delaminating the new render from the substrate.

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Poor key

Renders need a combination of mechanical key and suction to bond to the wall. Existing render surfaces, even though unpainted or coated, are usually 'plain face' and seldom have sufficient key to hold a new render.