every modern render manufacturer makes a cheap bagged sand and cement mix, then up the scale you get a played with thick base coat render that's more expensive but a better product, then you get the more expensive but thinner basecoats that are more like tile adhesives which are designed to go over cement boards or like you say scored but well adhered paint. These thin base coats are super sticky and flexible and together with being light weight will perfectly suit renovation situations where a full hack off is tricky or impossible. Then prime and top coat with an acrylic or silicone bucket render. Saying that though that job im on with the quoin stones, the back extension is block with fined down painted render that was impossible to remove, so we scored it heavily, then used two coats of parex maite with mesh in the first( their thin base render) and key combed this to receive a 15mm coat of parex monorex( their coloured render same as weber pral m). Don't know what weber's tile adhesive type render is called as i find their product list confusing at best and completely and utterly overcomplicated with too many products that overlap and do much the same as one another.
If weber was a restarurant off kitchen nightmares the first thing gordon ramsay would do is chop down the sodding menu to about 10 products!!