My name is Billy howsego I have been professionally plastering and rendering for 15+ years covering almost all aspects of building in between as well as heritage/listed buildings
If the bonding was put on recently then you should be able to scrape the surface back enough to take a finish if it’s been there for a while then same method either with the nose of a trowel or whatever tools at hand and lots of patience
Taking all the board down would be the best bet but if the boards are dry and in reletivly good shape i would just buy a bag of bonding and patch any holes or uneven surfaces so the wall can be pva’d and plastered after