Re-caulking a tub is 90 minutes, $10, and one genuinely odd trick: do it with the tub full of water. Here is the whole sequence, including the removal everyone rushes.
Riley Brand
New caulk over old caulk fails within months: it cannot bond to silicone residue, and any mold under the old bead grows straight through the new one. Slice along both edges of the old bead with a utility knife, then peel and scrape it out with a plastic scraper or an oscillating-tool caulk blade - a caulk-remover gel softens stubborn silicone if you let it sit per the label (often a few hours).
Get down to bare, hard surfaces: every rubbery crumb and silicone film must go, because nothing bonds to silicone except more silicone failure. Final pass: scrub the joint with isopropyl alcohol; treat any mold staining in the gap with a bleach solution and let it dry fully - completely dry, which after a bleach scrub means leaving it overnight with the bathroom ventilated.
Before caulking, fill the bathtub with water. A full tub weighs a couple of hundred kilograms and settles a few millimetres downward, opening the tub-to-wall joint to its widest real-world state. Caulk applied to a settled tub stretches as designed; caulk applied to an empty tub gets torn open the first time someone takes a bath - which is why the 'brand new caulk cracked in a month' story is so common.
Leave the water in until the caulk has cured per the tube (typically 24 hours for silicone), then drain. For shower-only stalls without a tub, skip this; the joint barely moves.
Use a sanitary/bathroom-rated 100% silicone - it carries mildewcide and stays flexible for years in wet service. Acrylic 'easy-clean' caulks are simpler to tool and paintable, but they harden, shrink, and mold sooner; in the splash zone, silicone wins. White, clear, or a grout-matched color are all available.
Run painter's tape along both sides of the joint, leaving an even 5-6mm gap - the tape is the difference between a factory-straight line and a freehand wobble, and it makes the tooling step foolproof. Cut the cartridge nozzle at 45 degrees, opening slightly smaller than the gap; you can always cut more, never less. Pierce the inner foil seal fully.
Run the gun at a steady speed with even trigger pressure, pushing the nozzle along the joint (pushing fills the gap; dragging rides over it), in one continuous pass per side where possible. Better slightly too little than too much - excess is what makes tooling messy.
Smooth immediately: a caulk-tooling tool, or a finger dipped in soapy water, drawn along the bead in one slow continuous stroke, pressing the silicone into the joint. Then pull the tape at once, at a 45-degree angle, while everything is wet - wait even ten minutes and the tape pulls a skin with it. One final ultra-light finger pass over the tape-line edges, and stop touching it.
Silicone skins within 30 minutes but needs around 24 hours to cure through - no showers, no splashing, tub water stays in (per the trick above), bathroom ventilated. Humid air actually helps silicone cure, but running hot water over an uncured bead ruins the surface.
To stretch the re-do interval from two years to five-plus: squeegee or towel-flick the joint line after showers (standing water is what feeds mold), run the fan during and 20 minutes after bathing, and hit any first mold speck with bathroom cleaner immediately - surface spots wipe off young silicone but root into old. When black spots live under the surface and no cleaner reaches them, that is the signal to re-do the bead, and now you know the whole 90-minute drill.
Sanitary silicone for the tub-to-wall and shower joints - it stays flexible and resists mold far longer in constant wet service. Acrylic/latex is fine for the dry trim: around the bathroom skirting, window, or above the splash zone, where paintability matters.
Practice and a steady pull - but honestly, just use tape. Two strips of painter's tape cost pennies, guarantee crisp edges, and remove the only hard skill in the job. Even pros tape fussy visible joints.
Either it went over old contaminated caulk, it is non-sanitary general-purpose caulk, or the joint stays wet (no fan, no squeegee, standing water on the ledge). Strip fully, use bathroom-rated silicone, and fix the ventilation - mold needs constant moisture more than it needs anything else.
Up to about 6mm as a simple bead; beyond that, press a foam backer rod into the gap first and caulk over it - the rod gives the silicone the right depth-to-width shape so it can stretch instead of tearing. Gaps beyond 10-12mm or moving more than that suggest the tub is inadequately supported, which is worth investigating before cosmetics.
