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Reach into the sink from above and run the blade of a utility knife through the bead of caulk, all the way around the rim of the sink. Work carefully so you don’t scratch the lip of the opening in the countertop. This caulk helps to hold the undermount sink in place but is primarily there to prevent water from getting between the sink rim and the underside of the counter. Although it will be a tight fit in the sink cabinet, this is safer and easier with a second set of hands helping you. While the second person holds the bottom of the sink, remove the several clips (often 4-6) that pin the sink rim against the underside of the counter. They will either be screwed or epoxied into place.  If they’re attached with screws, simply use a screwdriver to remove them. If they are stuck in place with epoxy, use a putty knife to scrape, pry, and wedge the clips apart from the underside of the counter. Once you remove the clips, the sink will be free to fall, so make sure it’s being held up by someone! Now that the caulk and the clips have been removed, simply guide the sink downward and out of the cabinet. If you are reusing the existing faucet and drain, remove them now. But it’s more likely that you’ll want to install new ones with your new sink. , but not the drain, in the new sink. Unlike with a top-mount sink, don’t install the drain before installing an undermount sink. But it’s easier to install the new faucet now instead of having to work from inside the sink cabinet. Installing a faucet is within the skill set of most DIYers, but the process differs based on the type and model of faucet. Closely follow the instructions that come with the new faucet. Use a silicone caulk designed for bathroom applications. Make sure the bead is continuous around the entire rim of the sink. Like the stuff you removed from the old sink, this caulk is mostly there for waterproofing, but also helps hold the sink in place. Cut a section of 2 in × 4 in (5.1 cm × 10.2 cm) lumber so it’s at least a few inches/centimeters longer than the width of the opening for the sink in the countertop. Lay this piece of wood across the opening. Then, while a second person lifts the new sink up into place from beneath, feed the bar clamp up through the drain opening of the sink so that one of its clamps holds up the sink from below. Secure the other clamp to the piece of wood and tighten it. Make sure the clamp is tight enough to make the some of the silicone caulk squeeze out between the rim of the sink and the underside of the counter. Wipe away this excess caulk with a damp rag. Your new undermount sink will come with support clips to place around the underside of the sink where it meets the underside of the counter. In some cases, these clips may attach with screws. If so, drill pilot holes and use a screwdriver to secure the clips in place. Otherwise, use the brand or type of epoxy recommended by the sink manufacturer. In most cases, the clips are adhered in place with a 2-part epoxy that hardens in about 10 minutes after being combined. Follow the product instructions carefully and apply the proper amount to each clip. Then press them into place as per the installation instructions for your sink. Even though the epoxy should be fully set in 10 minutes, it’s important to give the silicone adhesive time to cure fully. Leave the piece of lumber and bar clamp in place for a day before removing them. After that, you can put the drain in place and continue with the installation.  Like faucets, drain installations vary by type and brand, but the process is DIY-friendly if you follow the instructions carefully. You could rely on the epoxy to hold everything in place after 10 minutes and not wait the full 24 hours before proceeding, but this is not recommended. Be patient!

Summary:
Cut through the caulk that connects the sink to the underside of the counter. Remove the holding clips under the sink while supporting it from below. Lower the sink down and out of the cabinet. Install the faucet Apply a bead of silicone caulk around the entire top rim of the sink. Secure the sink in position with a piece of lumber and a bar clamp. Secure the included clips in place with screws or epoxy. Wait 24 hours, then install the drain.