Problem
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.
Additional materials put increasing stresses on the bond interfaces of existing materials.
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.
Solution
Treat, then apply weber.rend aid and suitable render
A detailed assessment of all areas of the existing materials and a choice of action minimising the risks based on that assessment is the key.
