Sink installers use silicone caulk to hold an undermount sink in place, as well as provide a waterproof seal. If the original installer did not attach the sink correctly in the first place, using any mounting hardware or allowing the caulk to fully cure, the silicone caulk seal will break and allow the sink to drop. You will need to reseal the break with more caulk.
Original Installation
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When an installer originally puts in an undermount sink, he needs to follow certain procedures to prevent the sink from separating from the underside of the counter later. If the installer rushes the job and takes certain shortcuts, the installation does not take as long. After the sink is loaded with dishes and water enough times, the sink starts to pull away from the counter, flexing the drainpipes and potentially leading to leaks in the pipes.
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Removing Old Caulk
Before you can apply new caulk to reseal your undermount sink to the counter, you must remove the old caulk from the underside of the counter and the top of the sink's lip. A metal putty knife provides a thin and sharp tool to scrape off the old caulk. If the caulk does not come off easily, you may purchase a caulk softener or remover solution you apply to the caulk. The softener solution starts separating the caulk from any surfaces it is attached to, allowing you to remove every last piece of caulk before you begin the resealing process.
Resealing
With the old caulk gone, you must remove the sink's drain assembly and drainpipes, as well as the faucet before you begin the resealing procedures. Removing these items reduces any extra weight pulling down on the sink, which may compromise the new caulk's seal before the caulk cures fully. You must keep the sink pressed against the underside of the counter while the caulk cures. Place a board over the sink cutout in the counter. Clamp one side of a bar clamp to the board and the other end to bottom of the sink through the drain opening. If the sink has two drain openings, use two bar clamps to hold the sink up.
After Sealing
If the sink has mounting hardware and it was not installed before, you need to drill holes for the hardware in the underside of the counter. You then apply a two-part construction epoxy to the holes you drilled before inserting the mounting hardware through the sink and into the counter. Another part of resealing an undermount sink properly involves allowing the caulk to cure completely before continuing with the installation process. Let the sink sit undisturbed for at least 24 hours before removing the clamps and installing the faucet, drain body and drainpipes.