Rendering on surfaces with poor key

Renders are not adhesives and require a combination of suction (drawing of the most liquid elements into the substrate) and mechanical key (texture or roughness) to bond to its substrate when rendering outside walls.

Common issues and queries you can face

Brickwork with poor key


Brickwork with the mortar joints raked back will provide sufficient mechanical key for rendering. However, if the joints are flush, a key has to be provided by other means.

Blockwork with poor key


As the density of blockwork increases, the finished surfaces often have much smoother faces. Raking the joints on blockwork will not provide sufficient key for rendering as the frequency of joints are considerably less than in brickwork.

Smooth concrete with poor key

Smooth concrete

Smooth concrete may offer the worst of all conditions for rendering, with the possibility of residual traces of release agents and both poor key and low suction.

Apply key coat and suitable render

Preparatory key coats are very heavily polymer-modified to 'stick' to poor key substrates and are finished with a pronounced texture to hold the new render.

  1. Prepare surfaces

    Ensure all areas are free from any residue that may interfere with the bond of materials to be applied.

  2. Apply key coat

    Apply weberend aid 2-3mm thick approximately 1m² at a time, textured immediately with a stipple roller to leave a deep, heavy, stipple key. Treat all elevations to be rendered. Cure as necessary.

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

  3. Apply render

    Apply and finish your choice of render as directed for the relevant materials.

    You can use a monocouche render such as weberpral M (or weberpral D in Ireland) or weberend OCR and a synthetic finish.

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