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.

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: GE Silicone II K/B Supreme

"Could older hardware cause this type of smell?" Certainly not. The smell of mold/mildew is unique. You are smelling what it is. Now you need to: 1. Find the mold or mildew, and 2. Eliminate the cause. The cause is moisture. The place you will find the mold/ mildew is inside the walls around your shower stall and the floor under the shower pan. Just about any shower stall built in the early 1990's is going to have mold. The construction materials and methods commonly used during that era have been discontinued. Poly plastic was often used as a vapor barrier, and sometimes a dreaded "double vapor barrier" situation occurrs. Also it is common that gypsum board products, like green board drywall, were used rather than a cement board product such as hardibacker or a proper mortar bed. I would gut the bathroom shower stall. The home inspector's report recommending replacment of the fiberglass shower pan, is a gentle way of phrasing "you need to gut the shower" I've never just replaced the shower pan in a tile shower. By the time the shower pan is bad, the tile walls are also bad. The shower pan is more robust and water tight than any tile work done in 1993. Can you open the drywall on the back side of one of the shower stall walls? That would be a good way to look for mold and still be able to abort the demolition work if you do not find it.

I realize these comments were from several years ago but I too have this issue and, I am not a slob and I live in Massachusetts where heat and humidity are limited to a couple months a year. However, I will begin to use a toothbrush to clean this area regularly. Although the mold has become visible it is very hard to reach. I never have known anyone to need a toothbrush to clean where the sink and counter-top meet but I am going to begin using a toothbrush regularly to reach this area. I would think twice before getting another under-mount sink. The black mold that builds up is disgusting and almost impossible to reach. I am going to replace the molding today with the mold resistant caulking someone mentioned in this thread and see what happens.