How to Caulk a Bathtub That Stays Sealed

01Remove every bit of the old caulk first

New caulk does not bond to old caulk; it bonds to clean surfaces. So the job lives or dies on removal. Run a utility knife or a dedicated caulk-removal tool along both edges of the old bead and peel it out in strips. Get into the corners and dig out the embedded remnants; a needle or an old chisel-tip screwdriver helps in tight spots.

Once the bulk is out, wipe the joint with a caulk remover gel or rubbing alcohol to dissolve the silicone film that stays behind, because that invisible film will keep new caulk from sticking. Scrape any mildew stains out of the gap too. If you leave old caulk in place and skirt over it, the new bead lifts at the first flex of the tub, which is the most common re-caulk failure.

02Clean, dry, and the silicone-not-acrylic rule

A wet or damp joint is the second failure mode. After removal, clean the gap, let it dry completely, ideally a few hours with a fan or after a day of no showering, and wipe it once more with alcohol just before caulking. Trapped moisture is what feeds the black mildew that blooms behind a fresh bead.

Use 100% silicone caulk for a tub, not acrylic latex. Acrylic ("painters caulk" or siliconized acrylic) is easier to tool and paintable, but it is not built for constant wet immersion; it shrinks, cracks, and lets water through at a tub joint within a season or two. A kitchen-and-bath rated 100% silicone stays flexible and waterproof for years. Many silicones also include a mildewcide, which is worth having in a shower corner.

• ·Use 100% silicone rated for kitchen and bath, not acrylic latex
• ·The joint must be bone dry; trap moisture and you trap mildew
• ·Wipe with alcohol right before the bead for the cleanest bond
• ·A silicone with mildewcide resists black staining in wet corners

03Tape the lines, lay the bead, and tool it

Run a strip of painters tape along the tub and another along the wall, leaving a gap the width you want the finished bead, about an eighth to a quarter inch. The tape gives you two crisp edges and saves a long cleanup. Fill the tub partway with water first if you want the most reliable seal, because the tub settles slightly under load and caulking it loaded keeps the joint from opening when someone steps in.

Cut the tube tip small at a 45-degree angle, load the gun, and lay one continuous bead along the joint with steady pressure. Then tool it: drag a wet fingertip or a caulk tool down the bead in one smooth pass to press silicone into the gap and shape a concave surface. Pull the tape off immediately while the caulk is wet, before it skins over, so the edges stay sharp.

04Cure time, and when caulk is not the real fix

Let the silicone cure before exposing it to water. Most 100% silicones are dry to the touch in 30 minutes but need a full 24 hours to cure through; rushing a shower onto a green bead breaks the seal you just made. Check the tube, but 24 hours of no water is the safe default.

Caulk seals a sound joint; it does not fix a moving or failing tub. If you are re-caulking the same gap every few months, the tub may be flexing because it was never properly supported, or water may already be getting behind the surround. A persistent leak at a tub joint can show up below as a water leak in the ceiling, and at that point caulk is treating a symptom. If the tub itself is cracked, stained beyond cleaning, or the surround is failing, weigh the bathtub replacement cost against another reseal, or look at a tub-to-shower conversion cost if you rarely take baths anyway.