To recaulk a bathtub or shower, cut out all the old caulk, strip the leftover silicone film, clean off mould and soap scum, and let the joint dry overnight. Tape both edges, fill the tub with room-temperature water, then run one continuous bead of mould-resistant kitchen-and-bath silicone. Tool it smooth in a single pass, pull the tape while the caulk is wet, and keep water off it for the full cure time printed on the tube.
Recaulking a tub is a job most homeowners can do well the first time, if they treat it as a prep job rather than a caulking job. When a DIY recaulk fails within months, the bead is rarely to blame. It went over old caulk, onto a damp joint, or came from a tube of paintable acrylic instead of silicone.
Here’s how we do it on site, which sealant goes where, what black caulk is telling you, and when the real problem is behind the tile.
Tools and materials you’ll need
- A hooked caulk removal tool and a utility knife
- Plastic razor blades for acrylic, fibreglass and enamelled tubs
- Gel silicone remover or isopropyl (rubbing) alcohol, and lint-free rags
- Unscented dish detergent and a non-scratch scrub pad
- Low-tack painter’s tape
- A dripless caulking gun
- Mould-resistant kitchen-and-bath silicone (see the table below)
- A rubber finishing tool or nitrile gloves
- A flashlight, and a fan or dehumidifier for drying
- Foam backer rod, only for gaps wider than about 1/4 inch (6 mm)
Which caulk is best for a shower or bathtub?
Inside the wet zone — the tub-to-tile joint, the shower corners and the base — the best caulk for a shower is pure silicone labelled for kitchens and baths, with a mould inhibitor. It stays flexible and doesn’t soften when it’s soaked every day. It can’t be painted, though, and nothing sticks well to old silicone, including new silicone.
| Sealant | Strengths | Limitations | Where it belongs |
|---|---|---|---|
| Pure silicone, mould-resistant | Flexible, waterproof, resists mildew | Not paintable; won’t bond over old silicone | Tub and shower joints, sinks, vanity tops, backsplashes |
| Siliconized acrylic (sold as “latex”) | Easy to tool, water cleanup, paintable | Shrinks; softens when constantly wet | Painted trim and dry joints outside the shower |
| Hybrid or polyether (MS polymer) | Flexible, paintable, low shrinkage | Few grades are rated for wet areas | Moving joints that need paint, such as window and door perimeters |
| Polyurethane | Tough; bonds well to concrete, masonry and wood | Messy to tool and slow to cure | Exterior masonry, concrete and siding joints — never the shower |
Silicone vs latex (acrylic) caulk
Plenty of tubes marked “kitchen and bath” are siliconized acrylic: latex caulk with a little silicone added. Inside a shower it shrinks away from the tile and softens, letting water in behind it. If the label says paintable and water cleanup, it isn’t silicone. Save it for painted trim, where it’s the better product.
Two more label details. Standard (acetoxy) silicone smells of vinegar while it cures, and that acid can stain marble and other natural stone, so use neutral-cure or stone-rated silicone there. And clear silicone shows every trace of residue and, later, mould; white or a grout-matched colour is more forgiving.
How to remove old caulk from a tub or shower
Removal decides whether the new bead lasts, so take your time here.
- Score both edges of the old bead with a utility knife held low against the tile. Use plastic blades only on acrylic, fibreglass or enamel.
- Hook the removal tool under the bead and pull. Silicone usually peels out in strips; acrylic tends to crumble.
- Soften anything stubborn: a hair dryer on low for acrylic, gel remover left on for its label time for silicone.
- Dig out old caulk, or grout from an earlier “fix”, that has been pushed deep into the gap.
- Shine a flashlight in. Dark, wet backing, soft wallboard or a tile that moves means stop and read the section below on when it isn’t a caulking problem.
Getting silicone residue off
New silicone won’t bond to a film of old silicone, even one you can’t see. Rub the joint with a plastic scraper and a rag dampened with remover or isopropyl alcohol until it no longer feels slick or waxy. On acrylic and fibreglass tubs, check the maker’s care instructions before using any solvent.
Clean off mould, soap scum and scale
Scrub black areas and the nearby grout with unscented dish detergent and water, then rinse. That’s what Health Canada recommends for small patches of mould, and it advises against bleach. Scum and mineral scale must come off too, because silicone bonds to whatever is on the surface, not the tile beneath.
Dry the joint completely
This is the step everyone rushes, and water trapped behind a new bead is how mould starts again from the inside. Skip showering for the day, leave the exhaust fan running and aim a small fan at the joint. Overnight is ideal. If a corner of paper towel pressed into the gap comes out damp, wait.
How to recaulk a bathtub, step by step
- Fill the tub with room-temperature water. The weight opens the joint to its widest, so the bead compresses rather than stretches when the tub drains; cold water can make the deck sweat right where the caulk needs to bond. It matters most with acrylic and fibreglass tubs, which flex more than cast iron.
- Tape the tile about 1/8 inch (3 mm) above the joint and the tub the same distance below it, pressing the edges down firmly, so the bead comes out an even width.
- Cut the nozzle at an angle with a small opening — you can always cut more — and puncture the inner seal. Most tub joints want a bead about 1/4 inch (6 mm) wide.
- Hold the gun at roughly 45 degrees and push the bead ahead of the nozzle at a steady pace, so the caulk is driven into the gap rather than laid on top. Run each wall in one pass; stopping mid-run leaves a lump.
- Tool it immediately, because silicone skins within minutes. Make one light pass with a finishing tool or gloved fingertip to leave a slightly concave surface, and don’t go back over it.
- Pull the tape while the caulk is still wet, slowly and at a low angle. Once the caulk skins, the tape tears the edge with it.
- Leave the water in and the fan running until the caulk has set according to the tube, then drain the tub gently.
How to recaulk a shower
A tiled shower or stall takes the same prep and technique. There’s no tub to fill, but the base should feel solid underfoot; if it flexes, the bead will keep tearing. Seal the vertical corners first, then the base joint. On glass enclosures, most makers want the frame sealed on the outside, with the weep holes in the bottom track left open so water drains back into the shower — check your enclosure’s instructions.
How long before showering after caulking?
Follow the cure time printed on the tube rather than a number from the internet. Many bath silicones list a time for light splashing and a longer one for full cure; wait for the longer one before a full shower. Cool rooms and deep beads cure more slowly, so give a cold basement bathroom extra time.
Why shower caulk turns black — and when the mould is behind it
Black spots on caulk are mould feeding on soap residue and body oils in a joint that never fully dries. The mildew inhibitor in bath silicone slows it down, but it wears out over the years. What matters is where the mould is.
- On the caulk: the specks lighten when scrubbed and the bead is still stuck down along both edges. Cleaning and better drying habits may be enough.
- Behind the caulk: the black won’t scrub off, or the bead feels spongy, lifts at an edge or weeps water when pressed. Water is getting behind the seal, and that caulk has to come out.
The usual causes: water getting in through a bead that has lost adhesion, a bathroom that stays humid for hours, and water sitting on flat ledges and sills.
Ventilation is the one most people underestimate. Health Canada’s guide to moisture and mould indoors says to use the exhaust fan when you shower and keep it running for at least 30 minutes afterwards. Hold a square of toilet paper to the grille; it should stay put. A fan that can’t hold it, or that vents into the attic, needs attention. That’s wiring work for a licensed electrical contractor; we can coordinate it and handle the patching and painting after.
Keep a squeegee in the shower and wipe the tub line and the bottom corners after the last shower of the day. It does more for a caulk line than any cleaner.
If mould comes back soon after cleaning or spreads beyond the caulk line, treat it as a moisture problem, not a caulking one. Health Canada recommends a professional assessment once mould covers more than about one square metre, or whenever it’s suspected inside walls or ceilings.
Caulk vs grout: which goes where
Grout is a rigid filler for the joints between tiles on the same flat surface. Caulk, or more accurately sealant, is flexible, and belongs wherever two surfaces or materials meet and move independently. Tile-industry practice keeps it simple: at a change of plane, use a flexible sealant, not grout.
| Joint | What goes there | Why |
|---|---|---|
| Between tiles on one wall or floor | Grout | Same plane and material, little movement |
| Tub or shower base to wall tile | Silicone | The tub moves with weight; the wall doesn’t |
| Inside shower corners | Silicone, matched to the grout colour | Two walls move separately |
| Wall tile to floor tile | Silicone | A change of plane |
| Vanity top to backsplash | Silicone | Different materials meeting at an angle |
| Painted trim and casing | Acrylic caulk | Dry, and has to take paint |
Grouted corners crack and get re-grouted again and again, when they should be raked out and siliconed. And caulk smeared over crumbling grout mid-wall hides the problem without sealing it; failing grout needs regrouting.
Signs it isn’t a caulking problem
Fresh caulk can’t fix a wall that is already wet or a tile that has let go. Before you recaulk, check for:
- Loose or hollow-sounding tile when you tap the bottom rows with a screwdriver handle
- A soft, crumbling or bulging wall at the tub line, which means the damaged board has to be cut out and replaced
- A dripping spout, a valve leaking behind its trim plate, or a ceiling stain below that appears even when nobody has showered. That’s plumbing. A stain that shows only after baths often traces to the drain or overflow rather than the caulk.
- A tub that flexes or creaks underfoot. Without proper support, any sealant will tear.
- A gap wider than about 1/2 inch (12 mm). Something has settled or moved, so find out what before sealing it.
Fixture swaps on existing shut-offs are handyman work; supply or drain lines behind the wall go to a licensed plumber. If you’d rather hand the job over, our caulking service covers full removal and re-sealing of tubs, showers, sinks and backsplashes. Loose tile, hollow spots and failing grout fall under tile and grout repair, often in the same visit.
Recaulking in GTA homes: what’s different here
- Hard water. Tap water across much of the GTA leaves white scale along the tub line, and silicone won’t bond to it. Descale, rinse and dry the joint, but keep acidic cleaners off marble and other natural stone, which they etch.
- Winter condensation. Shower air condenses on cold exterior walls and windows and runs down to the tub ledge. A window inside the tub surround, common in older Toronto houses, is the weakest spot in the room: seal its sill and frame with silicone. Cooler bathrooms in January also slow curing.
- Condo bathrooms without windows. The exhaust fan is the only ventilation, and in some buildings it ties into a shared system, so report a weak fan to management rather than altering it. A leak through failed caulk can reach the suite below; your declaration sets out who repairs what.
- Older plaster bathrooms. In pre-war Toronto houses and many of the 1950s and 1960s bungalows across North York and Scarborough, tile may sit on plaster or early drywall rather than cement board, and plaster softens when it stays wet. Very old caulks and mastics can contain asbestos, so have suspect material tested before grinding it out.
Check the use-by date stamped on the tube before you start. Expired silicone can stay tacky and never fully cure, and a half-used tube from the basement shelf is the usual culprit.
When to hand it over
One tub line is a reasonable weekend job. It’s worth calling in help for several bathrooms, natural stone, a glass enclosure, mould that keeps returning, or a deadline such as a listing. Our crews come from custom-home building, where a caulk line gets a close look on the final walkthrough, and we bring that standard to a single tub. Send a few photos through our contact page for a free, flat-rate quote, and see the winter home maintenance checklist for other seals to check before the cold.
Frequently asked questions
Wait for the full cure time printed on the tube, not just until the bead feels dry. Many bath silicones list a shorter time for light splashing and a longer one for full cure; use the longer one. A cool room or a deep bead slows curing, so allow extra time in a cold bathroom.
It rarely lasts. New caulk doesn’t bond properly to old caulk, and silicone sticks poorly even to itself, so water works its way between the layers. Mould on or behind the old bead stays trapped and shows through. Remove it all, clean off the residue, dry the joint, then seal.
Pure silicone labelled for kitchens and baths, with a mould inhibitor, for every wet joint: the tub line, the inside corners and the shower base. Use a neutral-cure or stone-rated silicone on marble and other natural stone. Keep paintable acrylic, usually sold as latex caulk, for dry trim outside the shower.
Only if it’s on the surface. If the spots lighten when scrubbed with detergent and water and the bead is still firmly bonded, cleaning plus better ventilation may be enough. If the black is under the surface, or the bead lifts or feels spongy, water has got behind it and the caulk has to be replaced.
Caulked, with silicone. The inside corners, the line where wall tile meets the tub or base, and the joint between wall and floor tile are changes of plane, where the surfaces move independently and rigid grout cracks. Rake out grouted corners and fill them with silicone that matches the grout colour.
There’s no fixed schedule. A well-prepared silicone bead in a bathroom with a working exhaust fan can last for years, while one laid over residue or a damp joint can fail within months. Recaulk when you see gaps, lifting edges, cracks, or mould that sits under the surface rather than on it.
Sources & further reading
Related service
Alan
Project Manager, FixitPlus Inc.
Alan has spent more than 20 years building in the Greater Toronto Area — custom homes from foundation to final trim, second-storey additions, and full-house renovations. He runs every FixitPlus job as the single point of contact, from the first quote through the final walkthrough, and writes the advice on this site from what actually happens on site.
- 20+ years of custom-home building across the GTA
- Project manager on new builds, additions and whole-home renovations
- Single point of contact on every FixitPlus job
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