Wet areas should use thin set , in your case glass tiles , and wet area , you should use 254 Platinum white from Laticrete or PROLITE white from Customs Products. They are the best thin set in the marked and is suitable for glass applications and wet areas. Mastic is just used for small tiles and non wet .

Hi Ryan. Yes, back-painted tempered glass. I don't recall the adhesive used for the niche. And I don;t believe the edges were painted, I think you are just seeing the white of the back of the glass from the sides.

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