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Discussion Starter #1 (Edited)
Dear experts,

I need to replace my broken under-mount bathroom sink. The existing one is held under the granite top with metal clamps. But also it is glued to the granite top. Or maybe it is some caulk. My question is whether I need that glue or caulk, as I cannot see what it is for. I don't know why I need waterproofing there, as water would never reach the granite top.

Thanks,
Dmitriy
 

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Seal the Sink in Place

For most bathroom undermount sinks, all that is needed to mount the sink to the granite is a silicone adhesive. Squeeze a very thin line around the sink cutout on the underside of the granite. The weight of the sink will compress the adhesive slightly, making it spread out, so a thin line is all that is needed. Lower the sink into place, upside down on the countertop's underside, then lift it straight up again. This will cause the adhesive to pull away in small "strings" from the counter to the sink. The air hitting these strings will make the adhesive bond stronger. Lower the sink back into place and leave it undisturbed for 24 hours to dry.

Using Mounting Clips

The majority of undermount sinks only need adhesive to bond them to a vanity counter, but some large or oversized bathroom sinks will also use mounting clips. This small metal clip fastens on posts to both the sink and the granite top. Epoxy is used to attach the posts to both materials, then the clip is slipped over them and a small nut is fastened down on each post to hold the clip in place. The epoxy will cure hard enough to fasten the clips onto the posts in approximately 10 minutes, but the clips still need a full 24 hours to dry before the counter is mounted into place.
 

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Discussion Starter #4
Thank you gentlemen,

Though I still don't understand why one needs silicone if there are clamps there. Also, how could weight of the sink compress the adhesive, if the sink is under the granite top, and I cannot turn the top over, as it is already installed. I am just going to replace a broken sink.
 

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How do you think you your going to seal water out between the top and the sink without the silicone?
 
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The silicone serves several purposes----it's an adhesive and will keep the sink stable if it gets bumped or a clip fails--

The silicone keeps crud from getting into the gap between the counter top and the bowl--eliminating a place for mold to grow.

Also keeps the cabinet dry--while you may not over fill the sink, it is likely that someone will splash water onto the seam---
 
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