If you have gaps greater than 1/8", then you can fill the gaps between the sheets and the corner joint gaps with modified thinset. You don't have to use mesh tape with the thinset, but you can if the gaps are large enough where the thinset my need a bit of backing.

After the thinset has cured, you treat the joints with HB. Coat the joints with HB, then embed the fabric, then coat again. This should lead to the pdf.

Thanks. I had read somewhere that I should NOT have thinset into the corners so I siliconed it instead. Should I peel out the silicone and use thinset or just proceed with thinset over the silicone with tape, and then HB?

I'm not sure if HB will bond to silicone. It may very well coat it just fine, but it might not "stick" enough to get the proper mil thickness. There's also the chance that it could stick during the application phase, but as the HB dries it shrinks a bit that shrinking could cause the HB to pull away from and separate from the silicon.

If you really want to keep the silicon, I'd recommend calling Laticrete tech department and asking them. They're friendly folk.

If it was me, I'd remove the silicon.

Where you want a flexible caulk or sealant in the corners is on your tiled wall. Rigid grout will crack and can allow water intrusion behind the tile. Flexible silicon in the corners will move with any house settling/wall movement, and it'll keep the tile/grout facade intact.

With a topical membrane like HB behind your tile, those corner cracks in the grout will be more cosmetic than anything else. But cracks in the grout can "hold" water and provide a start for mildew/mold, etc. Silicon in the corner can keep everything purdy and water-tight.

You can get caulk to color match and texture match (sanded versus unsanded) your grout.

This post was edited by mongoct on Mon, Sep 8, 14 at 14:51

Thanks. I removed the silicone. I found some really thin mesh to use in the corners. I'm not sure if the thinset will bond perfectly to very edges of the cement board where the silicone was, so the mesh will help and I won't have to wonder if the Laticrete will stick.
Neil

Because of not planning ahead, two walls of the alcove are about 1/4" below the old plaster wall with 1/2 cement board. I know that Hardie makes a 1/4" board and was considering putting that over the 1/2". Any reason that wouldn't work? Thanks for the advice.
Neil