Bathroom Wall Prep

We have installed 1/2"cement board on tub walls after stripping it to the studs(plastic vapor barrier installed behind cement board. Rather than attempt to handle the weight of one piece of cement board we installed two pieces on each wall(top and bottom). Despite all our attempts to get the wall flat prior to installing (house was built in 1906) the cement board surface is not completely planar/flat. Is there a joint material that we can trowel on to fill the cement board joints and low...

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