Rendering over existing rendered/painted backgrounds

Weber do not advise rendering over existing renders, site batched renders or other manufactured systems. There are too many unknown anomalies that would prevent an informed judgement. These specifically 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.

Rendering on to painted surfaces is not in keeping with good building practice. Delamination, crazing and cracking are highly likely outcomes in this scenario. If no other option is viable and a further coat of render is deemed paramount then the below must be taken into account. Weber 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 against the following measurables.

Additional coats of render can only be applied safely when the existing work is sound and has satisfactory adhesion over the whole of the area involved. Further rendering coats should not be applied over soft friable material or where the existing render is greater than 19mm thickness. Rendering which has a rough cast, pebble dashed, or similar finish will normally provide an adequate key. Scraped textures however, will require a pre application of weberend aid, stipple finished for key. Before commencing the render should be cleaned and treated with weber CL150.

A thorough inspection should include:

  • Locating cracks greater than 0.5mm which may be active
  • Arranging for measurement of cracks
  • Assessing the wall for movement
  • Sounding the whole wall areas to detect detached areas of render or substrate
  • Checking the construction details for effectiveness and suitability for increased render
  • thickness.
  • Random hammer tests to ensure the render is fully bonded to the substrate

The overall thickness of a rendered surface cannot exceed 25mm, this must be taken into account when planning to apply a new render finish. If proceeding with work, follow the below steps: 

  1. Brush the area down and remove any loose materials, clean with a mild detergent or biocide wash if needed, allow to dry.
  2. Re-set details utilising a 10mm profile bead, fix with weberend aid or weberend bead adhesive.
  3. Apply weberend aid at 2-3mm and stipple using a brush, stipple roller or loaded roller, allow to cure for at least 48 hours.
  4. Apply a 10mm pass of weberpral M (or weberpral D) or weberend OCR and finish accordingly.


For sound, existing, site batched renders that have existing non-Weber render applied, they should be checked for suitability of condition and be free from anything that will impair adhesion of a coating. Once this is done then a Weber decorative coating such as flexirend highbuild or webersil P would be in order.

Please note weber PR310 primer may need to be used prior the products listed above, please seek advice from Weber technical before application.

Contact us

If you have any questions or would like assistance, please contact our technical team:

Our technical team is available Monday to Friday, 8:30am until 5.00pm.

Click Compare to help you choose the right product for your project, if you need further assistance please email technical@netweber.co.uk

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