[Bud Cline]

In hindsight i should have added some additional studs to bring the wall to conformance.

Hindsight is always 20/20.

Cement board is not rated for 24" centers as far as I know. As you can see why. The wall must be flat and flat begins with the studs and the stud spacing.

Lastly i am tiling with 12x24" tiles and believe the choice of tile will add complexity.

You have no idea how right you are.

The studs need to be 16" on center.

 

[Bud Cline]

Does You/anyone think i will be somewhat succesful studding horizontally to create a nailing edge?

Don't see why you can't fir it out as long as you hold your spacing to not more than 16" and be sure to backup that seam with a strip.

Lastly how much imperfection can i mask with thinset and tape when i seal the seams?

Shouldn't have any imperfections to "mask". If your substrate isn't flat my friend you are just moving your problems up a notch. The bigger the tile the flatter the substrate must be.:yes:

 

[Bud Cline]

Sorry!

To "fir out" is to place strips of wood on top of other strips of wood in this case the studs. The studs are installed vertical. The firring strips would be installed perpendicular/horizontal to the studs and spaced every 16" on center. Making sure that the seam/juncture of the lower cement board and upper cement board have substantial firring behind it. This may require two firring strips at the seam/juncture location if using 1X2 firring.

Cement board is typically 1/2" in thickness but the quality control isn't that good these days. Fiber cement board is typically 7/16" thick even though they call it 1/2" also. Go figure.

Yes you can feather the cement board/drywall juncture but use thinset to do so. Probably shouldn't shim the drywall and create a curve you don't want. Filling it later is better in my thinking.

I would install the seam tape over the seams and wait until I was spreading the thinset to set the tile to fill the seams at that time. The cement board/drywall juncture may require a little advance fill.