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You dismantle it and replace the seal.
Install a ty fitting on the stack in the basement then run it to the sink. Put it low enough to account for a fall in the pipe, to allow proper drainage. Be sure to install a waste and vent pipe to code for both fixtures.
Take apart the drain and clean it up. Fit new seal, drain to bowl and new seal from slip tube to drain tube.
I'm a licensed plumber in Michigan and own my own business. We charge $119 for labor. This is for a replacement only. If you need the mounting flange replaced too or any repiping has to be done under the sink, this would incur extra costs.
A new disposal should have instructions with it. Disconnect the electrical supply and the dishwasher tube if it is connected. At the top of the disposal there is a locking ring that attaches it to the sink. There should be 3 tabs sticking out around the ring. They look like 1 inch long tubes. Rotate the ring counter clockwise and the disposal will drop down. Hold the disposal up while turning the ring. The neck sticking down through the sink should have 3 screws holding it in place. Loosen the screws and the plate they go through will raise up so that you can remove the snap ring at the bottom of the tube. With the plate and snap ring removed, push the neck out of the sink. Clean the surface of the sink and use plumbers putty or silicone caulk under the lip of the new disposal neck. Install the neck or flange in the reverse order you took the old one out. Same with the disposal. If there is a dishwasher attached to it, you will need to knock out the plug in the tube on the disposal. Just use a screwdriver and hammer to knock it out before you install the disposal.
Plumbers Putty. An oil based putty that is used to seal sinks and faucets down the the cabinet and between the faucet and the sink. It takes years for it to dry out and it never shrinks or gets completely hard so it keeps a seal and is easy to clean off when something needs to be done with the sink.
Normally you would use Plumber's Putty to seal the faucet to the sink. Some man made sinks can absorb the oil in the putty and darken in which case a bead of silicone caulking would work.
Determine where the leak is and fix it. If it is where the drain seal against the sink, take it out and put a ring of plumbers putty around it and reinstall it. Any other connection is going to have a gasket at the joint or it may be that a joint is not tight enough.
flange valves are useful because lets say in your sink the faucet is leaking you can use the flange valve to stop the water and repair the sink without having to turn the water for your whole house off.
Put putty under outer edge of faucet base and fasten to sink. Remove excess putty after fastening faucet.
Sink strainer nut broken or loose. Putty under sink strainer rotted away.
If it is real marble, the oil in the putty can leach into the marble and discolor it.
Pop-up valves may leak from four points: 1) the metal flange atop the pop-up valve, which seats within the ceramic opening of the sink; 2) the pop-up drain locking nut; 3) the pop-up actuator collar nut; and, 4) the P-trap compression fitting. Metal Flange. With all else tight, leaks from the metal flange remain internal to the drain assembly. That is, they do not drip out or create a mess but only keep the sink from holding water, even when a rubber stopper plugs the drain. To fix: Loosen the pop-up drain locking nut and elevate pop-up assembly, and then apply a generous strand of plumber's putty beneath the flange. Re-seat and tighten. Pop-up Drain Locking Nut. Leaks will drip, externally. Above the locking nut, a hard composite washer transfers tightening force to a black rubber seal, which insures water-tight integrity at the bottom of the sink. To fix: Tighten nut. If unsuccessful, replace black rubber seal. Re-tighten. Actuator Collar Nut. Leaks will drip, externally. To fix: Tighten nut. If stripped, replace. P-trap Compression Fitting. Leaks will drip, externally. To fix: replace plastic compression ring and tighten collar.
Mold
It's probably not condensing, but the seal between the sink basket and the sink is leaking.
In a nutshell, NO. An undermount sink requires a flat flange to bond to the underside of the granite or marble counter. Another consideration is the weight of a cast iron sink. Why would you want to do this in the first place?
You dismantle it and replace the seal.