An existing surface must be able to hold the rendering material in place even after it dries. Cement rendering, which is a mixture of lime, clay, cement, and sand, is best applied to cement surfaces. This can include sidewalks and retaining walls.
For other surfaces, such as painted brick, you might choose an acrylic rendering. This includes acrylic materials added to that cement mixture, which has a stronger adhesion. Acrylic rendering is often chosen for the exterior of brick homes, or for a crumbling chimney.