Rendering over existing render

We do not recommend rendering over existing render (site batched or manufactured systems) due to the large amount of unknown factors regarding suitability for new render. As explained in greater detail below, these unknown factors include the strength, bond, thickness and quality of the existing render, as well as any existing applied surface treatments which could affect the adhesion of the new render.

Instead, we recommend to either use a decorative finish over the existing render or remove and replace the existing render, however this may result in damage to the substrate.

Common issues and queries you can face

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Render coats are applied in successive layers

Traditional multi-coat render systems are applied in successive layers which become thinner and weaker towards the surface. Top coat renders therefore may be relatively weak and it is difficult to determine the bond strength of the existing layers of render.

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Stresses from additional materials

Each layer of render adds more weight to the previous layer(s), negatively impacting on the overall bond strength between layers of render, and between the render and the substrate.

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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 form a 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.

Poor key as an issue when rendering

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.