Seams or gaps that exist where you have PVC piping for drains or vents must be sealed. A quick and easy way to seal PVC is with silicone. The pipe must be dry before you apply the silicone; this allows the silicone to set up properly. To seal a PVC pipe with silicon you must first obtain one or more tubes of silicone, then prepare the PVC pipe.
Clean the PVC pipe's surface with a damp cloth or rag to remove dirt and debris.
Cut 1/4 inch from the tip of a tube of silicone caulk with the utility knife. Insert a nail or a small wire into the tip of the tube to puncture the seal.
Insert the tube of silicone caulk into the caulk gun. Press the push plate against the bottom of the tube.
Press the trigger to force the push plate into the tube of silicone.
Locate the seam or gap at the joint of a piece of PVC. Place the tip of the tube on the seam or gap that you need to seal.
Press the trigger until the silicone comes out of the tip. Slide the tip of the tube along the seam or gap to apply a bead of silicone.
Dip your finger in a bowl of water to get it wet. Slide your finger over the silicone to create a smooth texture.