Problem
Dirty deposits can form a weak intermediate layer that interferes with the development of the bond of newly applied render.
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.
Renders need a combination of mechanical key (texture or roughness) and suction (absorption of the most liquid elements into the substrate) to bond to the wall. Existing masonry surfaces seldom have sufficient key to hold a new render.
Solution
Treat, then apply weber.rend aid and suitable render
Assessment and preparation is of paramount importance. Preparatory key coats are very heavily polymer modified to `stick' to `smooth' substrates and are finished with a deep mechanical key to hold the new render.
