While attempting to repair a spot of water damage in the ceiling of one of my spare bedrooms (yes, the roof has already been repaired) I began to chip away at what I thought was Gypsum Type Wallboard. I figured maybe I would simply have to replace a small portion, patch & paint. I eventually chipped off what appeared to be a large chunk of Plaster... to uncover cement backerboard... only unlike any I've ever seen. Rather than a mix of fiber & cement, this is acutally Metal Lathe covered in real cement! I have never seen this before. My question is... how do I patch up these huge chunks of... I guess.. plaster that I have removed. The Cement Board is still in good shape, though there are rust spots coming through.. I obviously don't want the rust to bleed through. I was thinking about applying a skim coat of Thinset Mortar, followed by plastering. I was hoping to use Joint Compound to patch.. but I figure it won't bond too well to the cement/mortar. Am I on the right track here??