Home Improvement Questions? Ask a Handyman for Answers
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You can use 2" fiberglass mesh tape. You lay a thin layer of thinset over the tape to "seal" the joint. You have to be very careful to keep this layer thin or you'll create a hump which will throw off your tile. The primary reason for the thinset is to keep the tape in place when you lay your setting mud for the tile. You can even do without the layer over the tape if you're careful but it's easy to catch the tape with your notched trowel.
If you're using cement board it's not waterproof in that it will let water through. The trowel on membrane is never used for this purpose. If you want a backer that has a water proof surface and is waaaay easier to work with than cement board then take a look at DensShield by GP. It works just like sheetrock.http://www.buildgp.com/densshield-tilebacker-board
It's widely available but you may have to look around a bit for it since not everybody carries it. Here in MA/NH most Lowes (they sometimes call it DensGaurd) and Home Depot's carry it.
HardiBacker is more resistant to water penetration than cement board but it's still difficult to work. Take it from somebody who has remodeled bathrooms for over 40 years, DensShield will make this job a lot easier.
Ok, then my original answer covered this. No need for trowel on water proofing at the seams. I would caulk the seams with a vinyl caulk like Phenoseal before you apply the mesh tape if you want a tight backer but the water should never get that far anyway.
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