Problem
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.
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 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.
Solution
Apply weber.rend aid 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.
