I am a fabricator.

I have only seen this as an issue in cases where the customer wanted to undermount a drop-in sink or when the fabricator made a large (1/8" or larger) overhang.

An undermount sink has a sharp radius at the intersection of the vertical wall of the sink and the top flange. This leaves a very small cavity which is filled with caulk at install. Drop in sinks have a much larger radius and leave a large cavity at the sink/countertop interface if used as an undermount.

Properly done an undermount sink has a "reveal" where part of the top flange, typically 3/8 to 3/4", is exposed. Most OEM sink cutout templates have such a reveal. Unfortunately this approach exposes the caulk joint between the sink and countertop.

A small fraction of fabricators take the extra step to make a custom cutout that is just a 1/16" smaller than the actual sink. This very slight overhang covers up the flange and the caulk joint. It only works on stainless sinks because they have very accurate physical dimensions. It can be done on Cast Iron and other types of sinks but the overhang has to be a little larger to cover variations in the sink dimensions.

Thirteen years of porcelain undermount with granite. No mold.

Seems you could have mold in any sink cabinet if there's enough moisture trapped in there -- from pipes, dishwasher, wet sponges or cloths, whatever.

I always want an undermount in a kitchen if only because you can just brush things off the countertop into the sink.

Well, this came up in a recent thread and someone posted a photo of the top of an undermount sink after the counter was lifted up. IDK how often it happens, but the photo is enough to make you think twice!

Here is a link that might be useful: Thread with scary photo

Thank you, oldryder, for a professional's POV. And, chisue, for personal experience. I am finding that thread fascinating, jill314, so thank you, too. I'll let my friend know.

When I recently redid my kitchen, I had my own ideas, and damn the trends. As I've mentioned elsewhere, I have a smallish, galley kitchen with an eat-in area. Now, there are white upper cabinets, gray lower (mostly drawers), kiwi green laminate counters with Silgranit metallic gray drop-in sink, chrome knobs and accents, stacked satin rectangular white backsplash w/vertical stained glass mosaic accents in iridescent greens, and marbled gray linoleum floor.

Thank you so much for clarifying, Trebruchet! I so appreciate it. :)

Tre, what is polyester sealant? Is it ever appropriate to use anything besides silicone to install a sink? Is regular kitchen and bath caulk okay? I had an installer use gray siding caulk to install my undermount stainless sink, but I had him remove it and use silicone.

Don't worry about mold - the radon in the granite will kill it :-)

"Tre, what is polyester sealant? Is it ever appropriate to use anything besides silicone to install a sink? Is regular kitchen and bath caulk okay? I had an installer use gray siding caulk to install my undermount stainless sink, but I had him remove it and use silicone."

jellytoast:

There is no such thing as "polyester sealant" to my knowledge. What was used to bond these sinks was a knife-grade two-part polyester that is usually tinted and used for making seams and bonding built-up edges. I'd never use it for that either, especially in a kitchen, because of its lack of water resistance and its lack of UV ray stability. There are acrylic mehtelmethacrylates witch are much better suited. Notice on this same linked job how the bond of the front edge failed as well.

There are urethane caulks that would probably do a fine job sealing a sink, but I'm sticking with silicone. Paintable latex caulk (Polyseamseal) is never appropriate.

Before using any silicone caulk on stone you should do a test application first. Some silicones will stain some stones.