I have an old stone terrace house with most of the internal walls hacked back to bare stone. My intention is to render these walls then apply plaster board and skim. A number of the walls are not plumb and very irregular, in some cases there looks to be a 4inch difference between different parts or the wall. If I apply render this will result in it being extremely thick in places and a few mm in others. How is this situation tackled to create a nice flat finish or would it be usual to just work with the shape of the wall and accept it is not going to be plumb or completely flat.