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Weber Saint-Gobain - Official website of the company

Direct-applied rendering over existing render

The Codes of Practice do allow for further render coats to be applied over existing render materials, however, there are hidden dangers in the process.


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1. 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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2. Stresses from additional materials

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

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3. 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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3. 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.


Treat, then apply weberend aid and suitable render

Remove existing materials, provide a key with weberrend aid and apply full specification thickness render with either Monocouche or weberend OCR and synthetic finish.

Note: use weberend aid formulated with sulphate-resistant cement when rendering over substrates that may have a high salt content.

Solution with the following Weber products

Solution 1

weberend aid

Preparatory key coat for rendering

weber CL150

Masonry biocide which can be used on most exterior surfaces

weberend OCR

Pre-blended single coat application renders

weberpral M

One-coat, through-coloured, water resistant monocouche render

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