Caulk around a tub does not do its job if it is cracked, split or broken. Any opening that allows water to seep in between the bathtub and the wall exposes the insides of the walls to potential toxic mold growth, wood rot and mildew. Some contractors before they knew better or to cut costs used regular drywall near the bathtub. Drywall wicks up the water and moisture leaving the wall soft and mushy. If your wall is soft and mushy, you can just replace it, you will have to replace the wall. If the walls are strong and solid, recaulking is easy. Caulk should be removed and replaced every 2 to 3 years to keep a tight seal between the tub and the walls. Whether you are recaulking an existing tub or caulking a new bathtub for the first time, do it right to keep the inside walls and framing sealed off from water.
Removing the Old Caulk
Use pliers to get a good grip on the old caulk and pull it out.
Use a flat blade screw driver to dig out old caulk while being careful not to scratch and damage tile or fiberglass tub surrounds.
Very carefully use a razor blade to remove little bits of caulk and stuck on caulk.
If it remians firmly affixed to the edges of the tub, apply a commercially available caulk remover. Caulk removers are typically safe to use on any substrate, where they turn it into a gel-like substance that just needs to be carefully scraped away with a plastic putty knife.
Dampened a clean, soft rag with white vinegar to wipe down the areas, cleaning the gaps and edges that will be sealed with caulk.
Adding New Caulk
Fill the bathtub up with very warm water up to the overflow. Filling a bathtub with warm water has two benefits, the first is the heat from the water warms up the area to be caulked and a warm surface is more receptive to caulk. The second benefit is if you were to caulk and empty tub when you filled it up for the first time, it would weigh down and pull it away from the edges which will break it seal.
Insert a tube of caulk into a caulking gun. Cut off the tip of the tube with a utility knife and pierce the inner seal with a sharp nail.
Squeeze a long, continuous bead of caulk in one straight line into the gap between the bathtub and the wall.
Wear disposable rubber gloves and run your gloved finger along it bead. Or you can use the back of a plastic spoon to smooth and even out it bead.
Use a wet rag to clean up any spilled or messy caulk lines.
Allow it to dry completely before allowing it to get wet.
If you still have caulk left in the tub, place a nail into the open end and wrap the end in plastic wrap or place a bag over the end. Place a rubber band tightly over the tip. This will keep it from drying out and it will be fresh, free flowing and ready for the next use.
Choose a silicone caulk to use around a bathtub. Buy all caulking supplies from hardware stores or home improvement stores.