Buy a tube of silicone caulk and a caulk gun at a home-supply store or hardware store. Silicone caulk is more durable and water-resistant than acrylic or latex caulk.
Clean the pipe surface with soap and water and an abrasive pad. Scrub the surface of the pipe to remove dirt, grime and debris. Remove old caulking if the pipe has previously been caulked. Dry the pipe with a towel.
Cut the tip of the caulk tube at a 45-degree angle with a utility knife or razor blade. Place the caulk tube in the caulk gun.
Squeeze the trigger two to three times until the caulk begins to flow. Release the trigger to stop the caulk from flowing.
Place tip of the caulk gun near the pipe joint. Squeeze the trigger and apply the caulk to the pipe. Slowly move the tip of the caulk gun around the pipe until the joint connection is completely caulked. Release the trigger to stop the flow of the caulk.
Smooth the caulk line with your fingertip. Apply gentle pressure to help seal the caulk in place on the pipe.