Caulk vs Sealant: Which to Use in Bathrooms and Outdoors
Most caulk failures are the wrong product in the wrong place. There are four families in common use and they are genuinely different materials.
The four types
Acrylic latex (“painter’s caulk”) Water-based, paints beautifully, tools with a wet finger, cleans up with water. Little flexibility and poor water resistance. For interior trim gaps, baseboards, and crown moulding that will be painted. Never in a shower.
Siliconised acrylic Acrylic with some silicone added. Still paintable, somewhat more flexible and water-resistant. A reasonable compromise for kitchen backsplashes and around window trim indoors.
100% silicone Excellent water resistance, stays flexible for decades, bonds well to glass, tile, and metal. It cannot be painted — nothing sticks to it. For showers, tubs, sinks, and glass. Removing and replacing it is the only way to change its colour.
Polyurethane Very strong, highly flexible, paintable, and durable outdoors under UV and movement. Harder to tool, messier, and needs mineral spirits to clean up. For exterior joints that move: siding, masonry gaps, driveway expansion joints.
Choosing by location
| Location | Use |
|---|---|
| Shower, tub, sink | 100% silicone (mould-resistant grade) |
| Kitchen backsplash | Silicone, or siliconised acrylic if painting |
| Interior trim, baseboards | Acrylic latex |
| Around interior window casing | Siliconised acrylic |
| Exterior window/door perimeter | Polyurethane or exterior-grade elastomeric |
| Siding joints, masonry | Polyurethane |
| Gutters, flashing, metal roofing | Butyl or polyurethane |
| Around a toilet base | Silicone, and leave the back open — see below |
| Anywhere it must be painted | Not 100% silicone |
The joint matters more than the tube
Sealant works in tension and compression, and a joint shaped wrong tears the bead regardless of product.
- Width to depth ratio should be about 2:1. A 1/2-inch gap wants 1/4-inch depth of sealant, not a gap packed solid.
- Use backer rod in anything wider than about 1/2 inch. It is a foam cord you push into the gap so the sealant bridges rather than fills, and it prevents three-sided adhesion.
- Three-sided adhesion is the classic failure. Sealant bonded to both sides and the back cannot stretch, so it splits down the middle at the first movement.
Application
- Remove every trace of the old bead. A plastic razor, a caulk removal tool, then a solvent wipe. New sealant does not bond to old sealant, and this is where most reseals fail.
- Dry and clean. Silicone will not bond through soap film. Isopropyl alcohol on a rag; let it dry completely.
- Cut the nozzle small, at 45 degrees, and enlarge only if needed.
- Push, do not pull. Pushing forces the bead into the joint instead of laying it on top.
- Tool it once. A wet finger for acrylic, a dry tool or soapy finger for silicone. Repeated fussing breaks the skin as it forms.
- Respect the cure time. Silicone skins in 30 minutes and cures in 24 hours; do not use the shower before then.
Two specifics worth knowing
Mould in shower caulk that returns within weeks is usually growing under the bead, on a substrate that stayed wet. Cutting it out, drying the joint properly, and using a mould-resistant silicone is the fix; bleach on the surface is not.
Around a toilet base, seal the front and sides but leave a gap at the back. If the toilet ever leaks, you want the water to appear on your floor rather than soak the subfloor invisibly.
Storage
An opened tube keeps if you cap it airtight — a screw, not tape. Silicone and polyurethane cure by reacting with atmospheric moisture, so once air is in, the clock is running. Check the date on the tube; old sealant that never fully cures is a product problem, not an application one.
Movement capability, the spec that predicts failure
Sealant tubes state a movement capability as a percentage — ±25% is common on good polyurethanes and silicones, while acrylic latex may be ±5% or unstated.
It means the joint width the sealant can accommodate without failing. A 10 mm joint sealed with a ±25% product survives movement of 2.5 mm. The same joint with a ±5% acrylic tolerates 0.5 mm, which a timber window frame will exceed on the first seasonal cycle.
This is why exterior joints split even when the application was neat: the product was never rated for the movement. Match the product to how much the joint moves, and where movement is large, make the joint wider so the same percentage buys you more millimetres.
Priming and adhesion
Most sealants bond adequately to clean, dry, sound surfaces. The exceptions are worth knowing:
- Porous masonry, concrete, and bare timber often need a primer with polyurethane, or the bond fails at the substrate rather than in the sealant.
- Silicone will not bond to old silicone. There is no preparation that changes this — it all has to come out.
- Nothing bonds well to a wet surface, and “dry to the touch” is not dry enough on porous materials after rain. Give it a day.
- Powder-coated aluminium and some plastics need a specific product; check the tube for a compatibility list.
- Natural stone — marble, limestone, granite — can be stained by the oils in some sealants. Use a product labelled for natural stone.
Temperature and cure
Cure chemistry is temperature and humidity dependent, and every product has a stated application range, usually around 5–40 °C.
Applying below the minimum gives you a bead that skins slowly, stays soft, and picks up dirt. Silicone and polyurethane both cure by reacting with atmospheric moisture, so in very dry cold conditions a deep bead can stay uncured inside for weeks.
Plan exterior work for mild, dry weather with a dry forecast, and remember that a bead applied late in the afternoon may not have skinned before the dew falls.
Tools worth having
- A decent skeleton gun with a smooth ratchet and a thumb release. A cheap gun that will not stop extruding is most of what makes caulking frustrating.
- A caulk removal tool — a hooked blade for cutting out old beads.
- Plastic finishing profiles in several radii, which give a consistent concave bead that a finger cannot match.
- Low-tack masking tape for a crisp edge on a visible joint. Lay the tape, apply, tool, then pull the tape while the sealant is still wet.
- Backer rod in a couple of diameters.
Estimating how much you need
A 300 ml cartridge covers roughly:
| Bead size | Approximate length |
|---|---|
| 3 mm | 12 m |
| 6 mm | 8 m |
| 10 mm | 3 m |
| 12 mm | 2 m |
Buy one more than you calculate. Running out mid-joint means a visible cold joint where the two beads meet, and matching a partly-cured bead is never neat.
When to stop and use something else
Sealant is not a structural product and it is not a substitute for flashing, membrane, or a properly detailed joint. Two common misuses:
- Sealing over a leak in a roof or around a window head rather than fixing the flashing. It buys a season and traps water behind it.
- Filling a crack in masonry that is still moving. A structural crack needs diagnosing, not sealing; sealant just hides the evidence of what is happening.