There is also another GE Silicone II K/B Supreme that is "mold-free product protection helps keep the cured caulk mold- and mildew-free and looking clean for 10 years,with regular cleaning."

Here is a link that might be useful: http://www.caulkyourhome.com/ge-silicone-II-kb-supreme.php

I'm linking a discussion on this topic. Hope it helps.

And a link to one of the recommended products:

Lexel

Also, I've seen Poly Seam Seal recommended often.

The big problem is that none of the products will last indefinitely. The mildewicide eventually leaches out of all of them.

Here is a link that might be useful: Contractor Talk--Mildew Resistant Caulk

I never even thought of this potential problem. (so many things to think about with a kitchen remodel!) Is this something that I should buy from a hardware store to hand to the workmen installing an undermount sink or is this something they already know?

Fauguy, thanks for alerting us about this problem you're facing. You can't be the only one out there battling this issue.

Our installer ran the silicone back where you can't see it-- there is a little gap visible between the granite and the sink. We run a corner of a damp cloth around there regularly to keep it clean, and if the silicon's color changes, it won't be a problem. I didn't get how helpful this would be when the granite guy explained it, but it has been great. Just an idea.

Hard to believe it when a fabricator says he has never seen or heard of such a problem - probably because it never got back to him. Our Kohler cast iron under mount sink was installed by a professional fabricator about three years ago and the caulk is very, very moldy. Bleach does not get rid of it. Either does plain soap & water with a brush, which I tried first. The counter top is an exotic granite. Paid close to $10k for the counter top. The mold shows. I am now looking into some professional help with the problem. When it shows just standing in front it, it is a bit much.

babyboomer1001:

100% silicone is perfect caulk between a sink flange and the bottom of stone. Polyseamseal and other Latex caulks are not; they do not have the adhesive stickiness or flexibility of silicone.

It's best if you can pull your sink, clean the sink flange and stone bottom, recaulk with silicone and reinstall the sink. If this isn't reasonably possible, use a stiff plastic shim or plastic laminate to gouge the caulk out, clean the gap with acetone, and recaulk with silicone.

Ignoring a leaking sink flange can have catastrophic consequences if your top has been "rodded" or reinforced with steel. If the fabricator used the wrong rod and/or the wrong bedding the rod can rust and blow up your top. I've fixed about 8 of these so far and it runs around $1,000.00-$1,600.00.

We, too, have mold growing where the undermount sink meets the granite. Nothing will clean it. When I use a brush and bleach, black runs off but it never gets

Last summer 2017 we got all the old silicone removed from the 2006 install and it was pure black. Cleaned it with denatured alcohol and let it dry good. Then caulked in the GE Silicone II mold resistant, but has more of an opaque cloudy look to it instead of pure clear. But it's been about 10 months ago, and no mold groth on it. Only time will tell, but it looks soooooooo much better!

fauguy:

Unless you dropped your sink, cleaned the flange and stone bottom, siliconed with 100% silicone, and remounted your sink, you've done little. The caulk between the sink flange and the stone bottom does the work; topical silicone does little.

When this was installed in 2006, they put the granite in, and then mounted the sink under the granite which is held in place by metal brackts. Then they used silicone around the inside of the sink to fill the gap between it's rim and the granite.

So removing the silicone from the inside of the sink did get rid of it all. No need to remove the sink, since it is held in place by the metal brackts, also there all the plumbing and garbage disposal that would have to come out.

Fauguy, I would double check your sink is still adequately mechanicaly attached to your countertop. You mention a 1/3” gap between the flange and underside of countertop - that is pretty large. The countertop should be level and uniform, and unless the sink flange is damaged it should be relatively flush, with the gap being largely due to silicone.

Fauguy: Topical silicone does little work. If you have no silicone between stone and sink flange, you’re leaking. In addition to being unsanitary, you’re in danger of rusting any reinforcement rods in your top. They will split your stone. I fix this stuff constantly. Clips aren’t waterproofing.