You have just installed your new, gleaming bathtub or are in the process of refinishing an old bathtub and now it is time to waterproof the seams, one of the most important finishing steps. Without sealing the edges, water damage such as mold and rot can occur. To prevent water from seeping into the wall or floor around the tub, apply a bead of caulk along the joint where the tub meets the walls.
Remove any decaying, old caulk by carefully cutting it away with a paint scraper. Use a plastic, rather than metal, scraper if the tub surface is not scratch-resistant.
Scrub any remaining bits of caulk away with a scouring pad wetted with mineral spirits. Allow the cleaned surfaces to dry.
Mask the seam with painter's tape. Apply a strip of tape on the wall around the tub, parallel to and just under 1/4 inch above the seam. Apply another strip of tape around the edge of the tub, parallel to just under 1/4 inch below the seam.
Run a bead of caulk along the seam, between the tape strips. Use silicone or acrylic latex caulk. Silicone is more flexible but harder to work with. Acrylic latex is less flexible but easier to work with. Silicone is recommended for plastic tubs, which flex. Use acrylic latex on cast iron tubs, which do not flex.
Dip an index finger in rubbing alcohol so it is slippery. Glide the finger down the caulk line to push it info the seam and to smooth down the bead.
Remove the painter's tape before the caulk dries.