Faucets found on kitchen and bathroom sinks often have a base also known as a faucet deck plate. This plate or base serves to cover the openings where the water supply lines and bolts connect to the faucet and the handles. This is most common on faucets where all handles, the faucet, and drain pull are connected to the same piece instead of being separate units.
Applying plumbers putty is an important part of installing a faucet or when replacing a faucet after repair work. The plumbers putty serves to further secure the entire base to the sink top surface. This is similar to the use of plumbers putty sink drain uses to secure the sink drain and create a seal. The putty also prevents water from seeping under the faucet deck plate where it can cause water damage, warping, and mold.
First you must turn off the sink’s water supply underneath the sink or in the house main line if the sink lacks it’s own water shut off valves. Make sure the drain is open and open the faucet (or turn the water knobs on the faucet fully open) to drain any excess water. Do this step if you are removing the faucet to reapply putty or removing an old sink faucet for replacement. Credit: By Tomwsulcer (Own work) [CC0], via Wikimedia Commons
If you are removing the faucet and deck plate for repairing the putty or replacing the entire faucet you will need to loosen the locking nuts and washers directly under the faucet (underneath the sink itself towards the rear). If you can not find these nuts and washers you might be dealing with a top-mounted sink fixture. In this case lift off the faucet handle covers and remove the locking nuts now exposed. Keep in mind some top mount fixtures have small clips holding the handle covers in place.
Lift the faucet off, in some cases the faucet and handles will come off and leave the sink deck plate behind. In other cases the plate will come off as well. If the deck plate remains, use a flat head screwdriver to carefully pry the faucet deck plate off the sink.
Wipe off and remove all the old putty on the sink and on the underside of the deck plate. Allow the surfaces to dry if wet. Reapply a good amount of fresh plumber’s putty on the inner edges of the bottom of the deck plate. If there is a groove around the inner edge apply the putty in this groove. Put a small amount of plumbers putty around the holes in the sink itself (where the water lines connect to the faucet) for additional sealing against water seepage.
Carefully line up the deck plate, putty side down, over the fixture openings in the sink and press down firmly to create a good putty seal. Reconnect the locking nuts and handle covers. Double check to ensure the plate is secure and even. Turn the water supply back on and test both hot and cold water.