Caulk is a pliable material that goes on in a paste form, then dries to a hard rubber to seal out moisture and air. It's useful in many places but is particularly effective in bathrooms and kitchens. If the caulk is applied poorly, with wavy beads and gloppy surfaces, it may work as a sealant but could create an eyesore until addresses. Luckily, replacing bad caulk is one of the easier projects you can undertake. Getting that straight bead, however, can take a little practice, so try it out on some scrap wood before you start.
Cut out the old caulk by running your razor knife along the top and bottom of the caulk bead, separating the bead from the surrounding surfaces. Scrape your putty knife forward to remove the cut bead. Try to get it out in one piece, as that is easier than having to scrape it out in pieces.
Put water and a few drops of bleach on a sponge and run it along the exposed seam. Let it dry completely.
Put a tube of caulk in the caulk gun. Use the razor knife to cut off the tip of the tube at a 45-degree angle in order to make a hole about 1/4-inch wide. Push a long nail into the tip of the tube to break the seal inside. Squeeze the trigger of the gun to start the caulk flow. Press the release latch at the back of the gun to stop the caulk.
Set the tip of the caulk tube at one end of the joint. Tilt the gun to about 45 degrees and squeeze the trigger.
Drag the gun backward along the joinst as the caulk emerges, leaving a smooth, even bead of caulk. Go all the way to the end of the line, so the bead is consistent and unbroken. Hit the release latch to stop the caulk from flowing at the end.
Wet your thumb and press it lightly along the length of the caulk line, slightly flattening it and making it smooth and straight. Repeat for each joint. Let it set for 24 hours.