I'm doing a shower repair and learning a lot.

I think it would have been easier ultimately to do the whole wall, but I decided to only do the first 10 rows of tile from the bottom of a walk in shower that had damage up to that point. (Roughly 10x10 4.25 inch tiles.)

I removed the old stuff. Installed a 1/2 inch green board, then a 1/2 inch durock. (Simlar to what I removed.)

I taped up the seams. I left about half tile of old wall which just seemed logical to do, and that worked well (even though I've never seen that.) And, probably overkill, covered with Aquadefense.

However, in the top left corner of the repair, the new tile (not installed yet, just dry fit) will stick out about 1/4 inch. Its obvious the wall was/is not perfectly flat. There was something different about this area during deconstruction. I'm thinking it was the previous "fix" for this condition. Anyway, if I could remove material, I could get to a tolerable "lip". This is just a repair, learning not perfection. Someday I would like to gut/redo the whole room. So, how does one thin durock? Multitool? Sanding? Give me advice, or wish me luck.

 

I wouldn't do it - 1/2" is the standard for walls. Can you post pictures?

 

For starters, your 1/2" green board was likely too thick and was probably not needed. You likely should have shimmed the studs as needed so that when you put the cement board on, it would be the right depth for your tile to be flush.

So I'd suggest if you want the tile to be flush, start over, and get it right.

If you're not going to do that and want to do what you say, regardless of whether or not it's a good idea (it's not), then buy a $50 cup wheel for a 4 1/2" angle grinder and make a cloud of dust that it will take weeks to clean up from the house. Or have a helper squirt water on it as you grind in order to keep some of the dust down... and be sure you are plugged into a GFCI outlet.

 

It's never a good choice to have any kind of drywall inside a shower.

What was the prior damage?

Not what you want to hear but best to tear it all out and do everything correctly!

 

"...but I decided to only do the first 10 rows of tile from the bottom..."
Did you somehow install a waterproof membrane on the wall and seal it to the existing waterproofing?