undermount sink sealing

blgrosJune 15, 2011

We have had our undermount SS sink since our remodel 6 years ago. The only problem is the silicon seal around the edges gets mildewed and discolored after maybe six or either months, and we remove and replace.

What is the secret here? We're using the standard hardware store stuff that says clear, flexible, mold and mildew resistant, etc. Is there better stuff we should use? Is there a trick to the application?

Do others have maintenance free undermount sinks?


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Two different granite installers told me they only use 100% silicone. Silicone mixed with latex will get mildew. (I was complaining about my bathroom remodel when they explained this to me.)One of them said "You can't get it around here. we have to send away for it." I can't vouch for the accuracy of this, only reporting what I was told.

    Bookmark   June 15, 2011 at 10:06PM
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Horse pucky. Pure silicone should be on the same shelves that have all the other caulk.
Here's one:
GE Silicone II

    Bookmark   June 16, 2011 at 10:22AM
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Well that's good to know, but is it true that pure silicone won't mildew but the mixed one will?

    Bookmark   June 16, 2011 at 10:25AM
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It IS true that there are some "special" kinds of silicone that are sold to pro's only, but don't let that stop you from going to Lowe's or HD. (This is one of the blessings of having these mega companies around !! Without them, we would be unable to DIY unless we had loads of books, training and/or pro guides).

Around tubs, one places a "siliconized" latex which is designed to be temporary. Yes, it does allow / mold / mildew / whatever / to grow around its edges.

in the bathroom forum you all will find that bathroom renovators know a lot about the various types of silicone and the other products that are "latex"


    Bookmark   June 16, 2011 at 10:37AM
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well, I can tell you from personal experience that GE Silicone II, when placed at the grout line between the backsplash and granite, behind the kitchen sink (overmount), WILL definitely mold and mildew. I initially had that issue, then caulked it, which lasted about 2 years. Then I switched to the GE stuff, and put it on THICK, and it lasted about 2 years, and I recently scraped it off (it's ALOT harder to scrape off than caulk), and reapplied, except this time, I put a thin layer ON the granite between the backsplash all the way to the back rim of the sink. it's been about 4 months, and looks ok, but I"m not holding my breath.

I tried google searching it, looking for something that would truly work, and found that 'expert' plumbers, etc, all felt that silicone was not that good at preventing mold/mildew, and that regular latex caulk, I forget the actual brand, was the best. In the meantime, my new kitchen will have one piece of granite extending behind the undermount sink into the garden window, with no seam for at least 1 1/2' deep!

    Bookmark   June 16, 2011 at 11:38PM
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At the risk of sounding "self promoting" which I am not trying to be here (some dude gets all bent out of shape every time I mention that I've been in the Industry for over 33 years) = ANYWAYS......

Here's MY Opinion and feedback on MY Experiences:

I have had NO USE FOR GE SILICONE II - It is Ca Ca in my opinion - for what I need it for....

PURE 100% Silicone caulk (GE makes a couple of varieties as well as DAP and others) is what has worked FOR ME, however, I have had the BEST results (day in and day out) using acrylic latex caulks - such as Alex Plus. It's WAY easy to use, clean up is a breeze, and it lasts a good long time.

Another brand of caulk that I have used in the past with OUTSTANDING RESULTS is "POLY-SEAM-SEAL" . The Polyseamseal is less flexible than some of the conventional latex acrylic caulks, but it holds like iron and my customers over the last three DECADES have had no complaints....

just my opinion and .02 cents for what it's worth - again...

not trying to be "self promoting"



    Bookmark   June 17, 2011 at 10:21AM
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actually Kevin, your opinion is worth waaaaaay more than 2c
For the record i have NEVER thought that you were in the least self-promoting,
Always that you are being extremely helpful
thanks always for your words of wisdom

    Bookmark   June 17, 2011 at 10:34PM
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Yes, thats exactly what the pros said.... Poly seam seal! I just forgot the name. Someone brought up that there were different formulations, and they all said that the basic tub and tile one was what they used, iirc. Thanks for jogging my memory, Kevin!

    Bookmark   June 18, 2011 at 12:15AM
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Polly Seamseal. That's a nice name. If I name a pet after it maybe I can remember the name next time.

    Bookmark   June 30, 2011 at 9:28PM
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I have black-looking mildew on the caulking around my kitchen sink. Does anyone know how I can get rid or it or protect the caulking from mildew?

    Bookmark   October 16, 2013 at 6:01PM
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You may have a more serious problem than caulk.

Lay on your back inside your cabinet and make sure your sink is secured with mechanical fasteners. Wood shims/granite blocking polyestered in place bridging the sink flange does not count.

If there are no mechanical fasteners, disconnect your drain lines and drive a scraper between your sink and granite until the sink is free. Clean the sink flange and the bottom of the granite and install a Hercules Universal Sink Harness ($5.99 at Braxton Bragg)

Apply a nice fat bead of quality silicone to the flange and tighten the harness while keeping your sink reveal even. Spray the silicone squeeze-out with Windex and wipe off the excess. Use Acetone if you have to.

I have performed this repair at least 20 times. Never a callback, no mold or mildew.

If your silicone joint fails, the flange will dump water onto your reinforcement rodding if you have any. If this rodding is mild steel and set in polyester, it will rust, expand, and crack your granite parallel with your sink and front edge. A guy like me will charge you $800-$1,200.00 to repair it; no guarantees on performance or appearance. I have performed this 3 times and saw my worst one ever yesterday.

    Bookmark   October 16, 2013 at 10:08PM
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