How to Replace Old, Moldy Bathtub Caulk

Caulk is the primary line of defense protecting your bathroom walls from water damage, yet it is often the most neglected part of the room. Over time, moisture gets trapped behind the seal, creating a breeding ground for black mold that no amount of scrubbing can reach. When you see dark spotting or peeling edges, it is not a repairable issue; it is a sign that the seal has failed and needs to be replaced entirely. Executing this project well is less about the skill of your hand and more about the cleanliness of your surface. If you fail to remove every microscopic trace of the old material and soap scum, the new caulk will not adhere, leading to a repeat of the problem in months rather than years. Success looks like a clean, crisp, watertight bead that stays bright and mold-free for years to come.

  1. Strip Away the Old Seal. Use a caulk removal tool or a sharp utility knife to slice the old caulk away from the tub and tile. Pull the bulk of the strip out by hand, being careful not to gouge the finish of the bathtub.
  2. Eliminate Hidden Mold. Use a stiff plastic scraper to remove any remaining stubborn residue stuck to the tile and tub. Scrub the gap with a stiff-bristle brush and a mild abrasive cleaner to get rid of hidden mold spores.
  3. Banish Every Drop. Use a hair dryer on a low setting to force moisture out of the gap between the tub and the wall. Let the area air dry for at least two hours; any moisture left behind will prevent the new caulk from bonding.
  4. Clean Invisible Oils Away. Wipe the entire joint area down with a rag soaked in 70% isopropyl alcohol. This removes soap film and oils that are invisible to the naked eye but will cause the caulk to fail.
  5. Mark Your Perfect Lines. Apply painter's tape along the bathtub edge and the wall, leaving exactly the width you want your caulk bead to be. This ensures your final line is perfectly straight and professional.
  6. Lay and Smooth the Bead. Cut the tip of the silicone tube at a 45-degree angle to match your desired bead width. Apply a consistent, even bead, then run a finger dipped in soapy water over it once to smooth it out.