Sending some love From Buchanan Plastering Roughcasting @)----###xx
Drill and plugging (with umbrella shaped raw plugs) as a fresh key with a wire lath mesh to secure the old background that was solid to the wall. This original smooth render does not act as a good key as a top coat as it is a smooth render. This mesh gives the background a lifetime piece of security that can not crack/become loose to the wall or have any problems over years. This method can be used in almost every background including sandstone !