How to Caulk a Bathtub
To caulk a bathtub, first use a razor scraper to gently remove any old caulk around the tub. If the caulk is brittle and weak, you may be able to rub it off by hand. Use painter's tape to cover any surfaces around your work area that you want to keep clean and dry. Then, insert a tube of caulk into a caulk gun, and clip off the plastic near the tip of the gun. Remember to use caulk that is designed specifically for kitchens and baths. Start at a corner and hold the gun at a 90-degree angle to the seam that you are caulking. Pull the trigger to push the caulk out of the gun and move the caulk gun as it comes out so the width of your line is mostly even. Once you have applied a line of caulk, wet your index finger and run it along the seam where the line is. Push the caulk in as you move your finger to create a uniform bead that completely covers the gap between the tub and the tile. Repeat this process for every seam where your bathtub meets the tile, floor, and wall. Finally, let the caulk dry for at least 24 hours before using your tub.
Get a razor scraper to remove the old, existing caulk. This tool will cleanly remove existing caulk with a simple scraping motion. Make sure that the blade is plastic, as a metal blade can scratch and damage plastic bathtubs. If you are installing a new bathtub, there won't be any old caulk to remove! Skip this section and concentrate on applying new caulk. A utility knife is another option for a quick removal of caulk. Ensure that the blade is plastic. Although you can buy specialty caulk removers, these can damage plastic bathtubs. Fiberglass and acrylic tubs can also be easily scratched. Stick to plastic blades for these too. Use sharp strokes to scrape away the old caulk. Place the blade flush against the surface of the bathtub, and use small, quick strokes to scrape the caulk. This will remove the caulk from the surface cleanly. This motion will cause the caulk to flake away in long, thin pieces. Once you have gone around the bathtub removing old caulk, do a final check to find any spots you may have missed. Use tweezers to pry out any hard to reach caulk. Clean up the pieces of old caulk. Wipe up and remove any pieces of caulk that you can see. Then vacuum the tub to remove any leftover, hidden pieces. Wipe down the bathtub with denatured alcohol. Use an old rag to wipe down all of the areas of the bathtub where caulk was with denatured alcohol. This will help to clean the surface in preparation to apply the new caulk. If there are any particularly stubborn pieces of caulk that won't come away, then you can also use denatured alcohol to help remove these. Soak a rag in denatured alcohol and leave it resting over the caulk for 2 days. Then it should easily scrape away. Use a bleach solution to remove any mildew or mold. Mix ⅓ cup (80 ml) of bleach with 1 gallon (3.79 L) of water. Wear gloves and make sure the room is well-ventilated. Then, use a stiff brush to scrub the bathtub and the edges where the caulk used to be with the solution. Let this solution dry fully before applying the new caulk. Leaving it overnight and keeping it well-ventilated is best. If you have mold on your existing caulk, chances are it's coming from moisture behind the caulk. New caulk won't make your bathtub waterproof, so if you have a leak behind the shower, you'll still need to fix that. Purchase caulk made for use in bathrooms that is formulated for the type of tub you have. Hardware stores sell a variety of different types of caulk, so pick one that is for kitchens and bathrooms, or “tub and tile”. Caulk for bathtubs comes in either silicone or acrylic latex forms, and the best type to use depends on the material of your bathtub. For fiberglass bathtubs, silicone caulk is usually the best to use. It comes in a limited color palette and is difficult to smooth, yet is very flexible. For ceramic tubs, acrylic latex is recommended. It is easier to clean up that silicone and comes in many different colors, yet will need to be replaced slightly sooner than silicone caulk. Lay down strips of painter’s tape to get clean edges. Place strips of painter's tape just above all of the edges where you will apply the caulk. Next place parallel strips just below where the caulk will be, leaving a thin gap between the 2 strips. The gap in between these strips will be the width of the caulk. Although it depends on your style of the bathtub and edging, the gap between the painter's tape strips is normally around 0.375 inches (0.95 cm). Load the application gun with the caulk tube. Cut the tip off the caulk tube at a 45-degree angle. Push the stick on the application gun through the nozzle of the caulk tube to break the seal. Then place the caulk tube into the application gun, with the nozzle in the notched end of the gun. Hold the application gun loaded with caulk at a 45-degree angle from the edging. This means that the gun will be at an equal distance from either side of the corner where the caulk will go. The nozzle will be very close to the seam. Apply the caulk to the seam. Use a steady pressure on the application gun to carefully apply the caulk in the gap between the 2 strips of painter's tape. Move the gun smoothly around the entire edge. It doesn't matter if caulk gets on the tape, as you can easily peel this off after. Keep the application gun moving at a steady rate that matches the speed of the caulk leaving the nozzle. This will ensure that the caulk isn't too thin or thick. You can either pull the gun towards you or push it away from you as you apply the caulk. It works the same either way, so stick with what is comfortable. Smooth the caulk to form a concave shape. Dampen either a paper towel or a lint-free rag. Press it softly into the seam using your finger, and carefully run it along the caulk in one continuous line. You can also use painter's tape to press down the caulk. Just be sure to pull the tape away before the caulk gets tacky. Remove the painter’s tape. Peel away all of the strips of painter's tape from around the bathtub. This will remove any excess caulk and create a nice, straight line along the seam. Try and work as quickly as possible to remove the tape, as you also have to smooth the caulk again. Be as careful as possible when removing the tape, and try not to let it touch the caulk seam. Get rid of any small ridges left by the tape by smoothing the caulk again. Use a damp paper towel or lint-free rag again to smooth over the caulk. This will ensure that the whole caulk edging is seamless. Focus particularly on areas where 2 pieces of tape met, and these leave a small ridge. Let the caulk dry for at least 48 hours before using the tub. The specific amount of time that it will take for the caulk to dry will be listed on the instructions on the caulk tube. Keep the area well ventilated and dry as the caulk cures. You will be able to tell when the caulk is dry because the consistency will change to look firm rather than wet, and it will be completely dry to touch. If you use quick-curing caulk, it may be water-ready in as little as 30 minutes. Read the label carefully to be sure. 
Get a razor scraper to remove the old, existing caulk. Use sharp strokes to scrape away the old caulk. Clean up the pieces of old caulk. Wipe down the bathtub with denatured alcohol. Use a bleach solution to remove any mildew or mold. Purchase caulk made for use in bathrooms that is formulated for the type of tub you have. Lay down strips of painter’s tape to get clean edges. Load the application gun with the caulk tube. Hold the application gun loaded with caulk at a 45-degree angle from the edging. Apply the caulk to the seam. Smooth the caulk to form a concave shape. Remove the painter’s tape. Get rid of any small ridges left by the tape by smoothing the caulk again. Let the caulk dry for at least 48 hours before using the tub. 