Undermount kitchen sink gunk build-up

[Mike from Brier]

We have a large, deep stainless steel kitchen sink that is under-mounted beneath a granite counter top. If I remember correctly, the counter top installers used some kind of adhesive on the sink flange where it contacts the lower surface of the granite before they mounted the sink several years ago. Our problem is that we keep getting this black gunk building up in the seam between the sink and granite, and no amount of attention keeps it from returning. Last year we had a handyman come in who dug a bunch (if not all) of the gunk out of the seam, and then applied a high quality silicone caulk there to (hopefully) prevent it from reoccurring for a while. He said at the time we might need to do this every couple years, that this type of buildup is very common for an under-mounted sink. Keeping it as dry as possible by wiping the area down after each use was supposed to help, which we've tried to do, but the gunk just keeps building up and it's only been about six months since the "repair".

Is there a product we could install to the seam that might hold up better than the caulk he used without removing the sink in the process? Or would we be better off having the sink dropped and some type of better adhesive installed than the original installers did? And if so, what would you suggest?

By the way, there is zero evidence of water leaking under the sink anywhere, so that hasn't been an issue.

Thanks for any and all ideas!

 

[Jadnashua]

Some of it depends on the quality of the cutout the granite fabricator did on the slab. On my kitchen sink, when they cut the opening, there is NO lip...the granite essentially lines up perfectly with the vertical sides of the sink. THere's no rough under surface, and only a very small roundover on the bottom of the slab and the top of the sink. If you put a straightedge on it, it would not show a gap except for a very small joint right between the sink and the top. As a result, that portion of the slab is smooth and polished, and there's only a very small caulked seam to keep it waterproof.

With the rough slab surface, it will be almost impossible to keep it clean, dry, and debris free.

 

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[Mike from Brier]

Thanks for the reply. I checked the cutout as you mentioned, and it seems to be just as you indicate it should be. No lip, only a very small rounded V-groove between sink and slab, and when fabricated the slab was smooth and polished along the entire seam. Upon closer examination, the worst area is about 12 inches either side of the faucet, which we had corner mounted instead of in the center. Of course, this is the area that seems to get the wettest, and so it gets wiped down post-usage with a dry rag the most.

At this time, the rest of the joint looks fine.

So, caulk (which tends to deteriorate and discolor over time) in the seam only, huh? There's no magic plumber's epoxy type substance used in applications such as this?

Side note: If we had to do it over again, we wouldn't mount the faucet where we did...it looks contemporary and allows good access to the whole garden window behind the sink, but it's not as functional as a centered one would be.

 

[Jadnashua]

Looks like the same granite I have in my master bath. That's good and bad, Kashmir white tends to change color if it gets wet. I also chose to mount the faucet on the right, and would probably still do it given a second chance!.

I think what you're seeing is just one of the consequences of that style. I have not tried to find an epoxy to fill in that seam. Might work if you could dry it out and clean it up well first. WHen the sink was installed, a seam of silicon was spread on the underside, then the sink was clamped in place via the T-bolts the fabricator milled into the slab. This keeps it from leaking through that seam. I suppose if there were enough there, it could have been tooled flush with no gap, but it wasn't.