How to Fix Large Cracks With Caulk in the Bathroom

Remove all of the old material from the joint before you apply a new layer of caulk. You cannot caulk on top of old material, as it is dirty and already compromised. Instead, you need to make room for fresh caulk. Cut out all cracked joints with a utility knife. Cut out the entire joint, as you cannot patch with caulk because the color will be too different. Instead, you need to fill the entire joint.

Fill the empty void in the joint with foam backer rod for large cracks. Caulk shrinks excessively when it is applied to too wide a joint, and you can help avoid that issue by backfilling the joint. Cut the pieces to fit, using a utility knife, and fill the joint until you are roughly 1/8 inch away from filling it completely with foam.

Trim the tip of the caulking tube down with the utility knife until it is just barely able to slide into the joint you are filling. Load the tube of caulk into the caulking gun, and insert the tip of the tube into the joint. Pull the trigger on the caulking gun and fill the joint so it's just barely flush. Move the tip along the joint to fill it all.

Clean off any excess from the face of the tile, using a damp sponge. Do not allow excess water to accumulate on the surface of the joint for horizontal joints, as it will impede the drying process. Lightly press the sponge against the face of the joint to remove water, and smooth the joint with a wet finger if you need to.