just filling in the chipped sections with "dry wall adhesive" because I know this will form a better bond and sets rock hard. Tried to scrape a section of it and its stuck firmly to the sandstone. ;-) I've just checked it and rubbed it down and its as smoothe as my backside when I was a baby. Will give it a coat of primer when the weather looks ok and a gloss finish coat.
I'll take some pics of it before primer and the gloss finish.