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After I hang the drywall there is some gaps between the fixture and the drywall board (yes, I'm using greenboard that will be tanked/redgard/kerdi/etc, no backerboard in Brazil), what is the best way to caulk those areas?

I thought about using either PU foam or some flexible mastic (main component is polyisobutylene) that remains flexible (and I'm not sure it fully adheres to drywall). Silicone might also be a choice but some of the voids are to large to use it (about 10 mm).

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You probably want to start with tanking tape, and use caulk for the final seal.

https://youtu.be/IwR-vS4xc9g?t=9m15s

  • I'm familiar with that video, from what I understood the gap was already sealed with some "Plumbers Gold" (seems to be some kind of PU sealer). – Luiz Borges Apr 14 '16 at 18:39
  • Yes, if you want to fill the gap to better support the tape, do so. Ultimately the water barrier is the tape and however you seal the tape to the fixture, though. – isherwood Apr 14 '16 at 19:05

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