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Thread: Need to caulk under-mount sink?

  1. #1
    DIY Senior Member k9mlxj's Avatar
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    Default Need to caulk under-mount sink?

    I have a granite bathroom countertop that comes with undermount sink I am going to install.

    I notice that there's no caulking along where the undermount sink meets the granite top. Do I need to add caulking around it to avoid water leaking thru' that area to the outside when water splashes around? If so, I think it'd be along the inner porcelain surface?


    I had added silicone caulking along the inner porcelain surface of another undermount sink where it meets the granite countertop before. Black mold still develops over time along that line, so I need to clean that up manually with a thin wooden stick (can't use heavy chemicals on granite...). Something I don't enjoy doing frequently... .


    I just wonder which is the best way--and to avoid black mold from developing.
    Last edited by k9mlxj; 02-13-2011 at 10:15 AM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The 3 I've installed in my home, I spread a good glob of silicon on the flange before attaching the sink to the underside of the granite. This does two things: seals it, and helps to hold it in place. The granite fabricator I use mills T-slots in the granite, and you use T-bolts and clamps to hold the sink in place. If the sink is already glued in some manner to the granite, then it probably isn't an issue. But, if it is just clamped with nothing there, I'd take it off and then silicon it in place with the clamps.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Plumber jimbo's Avatar
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    MUST have caulk between the sink and the underside of the granite. After the sink is clamped in place, before it sets up, rub off any caulk which has squeezed out. Use wet finger and/or damp cloth, and use denatured alcohol for silicone caulk. You should be left with a barely visible caulk line, minimizing ongoing cleanup.

  4. #4
    DIY Senior Member k9mlxj's Avatar
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    > rub off any caulk which has squeezed out.

    I don't think for the last countertop+undermount sink I did a very good job rubbing off all of the caulking that squeezed out, or perhaps that area was hard to get close enough to. Now black mold develops along that edge.


    Would that pose a health hazard? I saw flies around the black mold once or twice (I have a separate drain flies problem from the toilet--but don't know if this is also providing a feeding ground for the drain flies as they come up from the drain). I wonder if I need to take out the undermount sink and redo the caulking again because of that black mold. Or black mold is unavoidable along that area regardless of how little caulking shows up along that area.


    There are 3 T-bolts and clamps that come with the set to hold the undermount sink, so the caulking is mainly to seal off the sink from water going thru' along that edge.
    Last edited by k9mlxj; 02-13-2011 at 10:02 PM.

  5. #5
    DIYer, not in the trades LLigetfa's Avatar
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    Good quality tub and tile caulk contains mildewcide but still it is best practice to keep it away from the inside of the rim.

  6. #6
    Plumber jimbo's Avatar
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    It is necessary to CLEAN tubs and showers and sinks regularly. Use appropriate disinfectant/cleaners. Use moldex at the FIRST SIGN of a mold problem. Don't wait until it is a bigger problem.

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