Hi all, hoping someone can offer me some advice. I recently purchased an old solid stone cottage slate to be precise. On the rear of the property is an extention built 40 years ago. Therefore what was an external stone wall is now an internal stone wall. This was originally covered in concrete render and still is. It was then covered in gympsum plaster and painted. This has since pealed off and some of the concrete has turned to dust. I have chipped away the loose stuff but would never remove all the concrete. If i leave it to dry can I apply lime putty over it to paint over or reapply plaster? Little unsure on how to treat this. To more the other side of the wall is of traditional construction so lime putty and lime wash. Photo added for reference. Thanks in advance, Mark.