Kitchen sink drains, just like every other drain, can leak. If you find lots of water underneath your sink and your faucet isn't leaking, then you need to seal your kitchen sink drain. Older caulks, such as plumber's putty, can degrade over time, causing water to seep under the drain. By removing the drain flange, you can reseal the drain and keep it watertight.
Place a bucket underneath the curved portion of the sink drain. This portion of the drain pipe is called the P-trap. Loosen the couplings at either end and pull the P-trap out of the drain. Empty the water inside the P-trap.
Unscrew the retaining nut holding the sink drain in place on the underside of the sink. This can be fairly tight. Spray the ring with penetrating lubricant and allow it to soak in. Remove the ring with the pliers by unscrewing counterclockwise.
Tap the underside of the drain in the sink above with the rubber mallet until it pops out.
Clean out the area around the sink drain. Scrape away any old putty or caulk with the putty knife. Use the kitchen cleaner and rags to scour the drain area.
Turn the kitchen sink drain flange over so the bottom is pointing up. Insert the silicone caulk into the caulk gun. Trim off the end to the 1/4-inch marker. Apply a bead of silicone caulk around the underside of the flange. Press the flange into the bottom of the drain, twisting slightly as you do so to help spread the caulk.
Have an assistant hold the drain flange in place. Position yourself on the underside of the sink. Thread the retaining nut onto the threads on the underside of the sink. Tighten with the pliers.
Wrap the threads on the tail piece and the sewer inlet pipe in the wall with Teflon plumber's tape. Insert the P-trap back in place in the drain. Tighten the couplings.