Well, this is the first I've ever heard that anyone has had mold or mildew with an undermounted sink (I assume that's what you mean). I live in MD, so I'm in the same region.
I wonder if what she's really talking about is the "gunk" that can collect under the countertop rim when you have a negative reveal (i.e., the countertop extends past the sink sides). This is not a problem w/positive or zero/neutral/flush reveal. However, as long as you're fairly good about wiping down the underside of the countertop when wiping down your sink, I don't see how this would be a problem.
Sink Undermount Options
There are pros & cons for each type of reveal:
Positive Reveal. The sink shows; granite cutout is slightly larger than sink (some people say it "frames the sink")
Pros: Easier to clean b/c you can see the gunk and can easily wipe it off (it only gets nasty if you leave it there)
Cons: Silicone (caulk?) is visible, but if they use clear you won't see it when it dries
Negative Reveal. The granite overhangs the sink; granite cutout is slightly smaller than the sink
Pros: You cannot see the gunk buildup or silicone/caulk
Cons:
You cannot see the gunk to clean it. (Since it's a working sink, gunk will get underneath the overhang and you won't know it unless you look)
Dirty water/food can splash up & under where you cannot see to clean it. It's difficult to see underneath w/o leaning pretty far over & into the sink.
Dishes/glasses have been known to break b/c when you lift them out near the edge of the sink the dish hits the stone counter & can break
For softer stones, pots & pans can chip or ding the edge of the stone around the sink if they bang against those edges
The granite often isn't polished under the reveal so things are more likely to "stick" (I don't know if this applies to Soapstone)
Won't know if caulk needs to be repaired (possibly allowing water to get into & damaging the sink base)
Zero Reveal or Flush. Sink & granite are flush or even; the granite cutout & sink are the same size
Pros:
Easier to clean b/c you can see the gunk
No platform over or under for the gunk to collect
Cons:
More difficult to do perfectly
Silicone is visible, but if they use clear you won't see it when it dries
Many granite installers prefer a negative reveal b/c they don't have to be as careful during install to match the template exactly as you do w/a positive or, especially, a flush reveal. [Note: This is definitely not true of all installers. Many are very conscientious and do a very good job.]
And that may be the problem...poorly done & installed negative reveal + not realizing how to clean.
So, do your homework and get a good installer!
BTW...just b/c you have an expensive house doesn't mean you have quality products or installation...I know as we live in an area w/very expensive homes (DC-metro area) and I've seen cheap cabinets, poor installation, & very...
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Green and gold finishes
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